“Oh my god, fuckkkkk!”

Heather collapses on top of me. A sheen of sweat glistens on her forehead and her dark hair is stuck to her face. She rolls off and lies on the bed beside me.

“Jesus, Mark, that was good.”

Her giggle is infectious and I smile as I lean across to kiss her on the mouth.

“You are so fucking delicious.”

That’s no flattery. She is. I fell for her the first time I laid eyes on her, whilst she was serving canapés at my friend’s exhibition opening at the gallery.

When she asked me if there was anything I fancied, I made her blush when I smiled and whispered, “Yes… you.”

It was a cheesy line but whether it was that, my well-tailored suit, my brooding good looks (her words, not mine) or the fact I appeared to have plenty of cash to throw around, one way or another, she spent the night in my bed. She admitted that night that she’d a thing for older men and while I didn’t feel particularly old, being twenty-eight at the time, the age difference was always there in the background while she was a student.

Four years later, however, she’s graduated and has a job as a marketing assistant in a publishing house, but she’s still as addicted to riding my cock as she was as a nineteen-year-old student.

She brings my reminiscences to an abrupt stop when she whispers she has a favour to ask me.

She’s always felt a little bit uncomfortable that she just moved into my apartment. She told me once she felt like a kept woman sometimes. I’ve tried again and again to reassure her that it is her home too, but I don’t think she ever quite believes me. I guess she feels a bit uncomfortable.

She kneels on the bed and brushes her hair back from her face. I love her long hair. The memory of me fucking her doggie style and pulling on it like reins causes my cock to twitch. The fact she’s still naked and her pale pink nipples are pointing right at me may also have something to do with it.

She has a great figure. Her boobs were the first thing I noticed about her. The buttons on the cheap white blouse she wore that night seemed to be under strain. They aren’t massive but on her petite frame, they seemed bigger. I’ve told her many times that they’re the perfect handful.

“Darling,” she begins. “You know Katie has to do a work placement as part of her course.”

I nod. Of course I know. I’ve had to pull quite a few strings with some contacts to get her the internship in the first place.

“Well, she starts on Monday but she hasn’t found anywhere to live yet. Do you think she could stay here for a bit?”

I sigh. I don’t mean to, but Katie is a handful. She is the polar opposite of Heather. She’s the mouthy, lazy and rebellious younger sister. She’s four years younger and everything Heather is not. While Heather likes to please and goes out of her way to make sure everyone is happy, Katie just thinks about herself and rarely if ever does Heather’s or her parents' feelings enter her head.

“I promise it won’t be for long.”

Her big brown eyes bore into my soul.

“And I promise she’ll behave.“

She lowers her face down to brush her lips against my swelling cock. She knows my weak spot. She knows that when I’m getting a blowjob my brain is bypassed and I’ll agree to anything.

“Promise?” I whimper. The sensations of her tongue licking the underside of my cockhead make my eyes roll in my head.

She mmmhmms a yes as she sucks my cock into her mouth.

“Fuck, you are such a bad girl,”

She lifts her head, a trail of drool connecting my cock to her bottom lip.

“Is that a yes then?”

Her eyes stare up at me as she lowers her head and takes my cock back into her mouth, never breaking eye contact as she swallows half the shaft. My back arches, my fingers grip the sheets.

“Fuck… Yes... Yes!“

I lie in bed and watch when Heather finally gets up to shower. Sunday morning sex is the best. The sway of her hips as she heads to the en-suite bathroom makes me wonder if she’ll be up for round three when she gets out. My only niggling doubt is if having Katie in the apartment will put a dampener on our sex life. Heather is quite shy and while she can be quite the animal during sex, if she thinks people might hear her, she’ll freeze up. Despite staying over at her parents' several times over the past few years, Heather still refuses to have sex with me in their house.

I’ve only just taken my coat off on Monday evening when there’s an insistent ringing at the door. I force my mouth into a smile at the sight of the blonde-haired girl on the doorstep.

“Hi, Katie.”

“Hi, Mark.” She slips past me into the apartment. I close the door and watch her hang her coat up on the rack. She has the same petite build as Heather, the same cute smile but sparkling blue eyes to go with her blonde hair rather than her sister’s dark hair and brown eyes.

“So, would you like a cup of tea, or a coffee maybe?” I offer as I follow her into the main living room/kitchen area. She is wearing a pale grey blouse and navy skirt and I realise this is the first time I have seen her looking grown-up. Usually, it’s jeans and tee shirts or ridiculously short skirts or dresses.

We settle on coffee and as I plug in the machine and prepare the cups, she drops her bag in the corner and slips off her heels. I watch her slide onto one of the stools at the island and am treated to a glimpse of stocking top as she crosses her legs.

Thigh highs are another of my weaknesses so to avoid temptation, I position myself on the other side of the island and we stare wordlessly into space for a few seconds as we both take the first sip of coffee.

“So how was work?” we both ask, and both stop and giggle simultaneously.

When Heather comes home, she’s surprised to find me doing paperwork at the table while Katie dances around the kitchen making a Thai green curry. Once Heather is settled with a glass of red wine, she interrogates Katie on the internship and how it’s all going before moving on to plans for their Mum’s 40th birthday.

I never realised their Mum was that young. I've always known she looked pretty fit and toned, but I’m still taken aback at how young she is.

“You can see Mark trying to do the maths,” Heather giggles.

“I bet you never told him you were an accident and Mum and Dad only got married cos of you.”

“Fuck off, Katie.”

Katie blows Heather a kiss and does a mock curtsey. I glance warily from one to the other. I can't tell if this is banter or serious. Anything involving Heather and Katie can be unpredictable. Finally Heather smiles, throws a tea towel at her and the tension passes.

It turns out that her Mum doesn’t want a massive fuss and it’s just going to be a meal in the house with caterers doing the food and then a bit of a party afterwards, but no DJ or marquee, despite Katie’s protestations.

Their Mum is so sweet and always seems to go out of her way to make me comfortable and feel at home any time we go down to visit. She’s a primary school teacher and I think that sometimes she forgets that her daughters are grown women the way she fusses after them, worrying that they will get home safe and that they are eating ok. I used to find it irritating how Heather would have to phone her to let her know we were home again after a visit but now I find it endearing.

The three of us quickly fall into a routine. It turns out that Katie is a really good cook and has taken it upon herself to cook almost every evening. We both get home around the same time and I quickly start to look forward to the wind-down from work with Katie cooking and me topping up her glass of wine. Occasionally she trusts me to slice up an onion or two, but most of the time I just sit there and regale her with stories from the office. The subject of Katie staying only temporarily and her finding her own place never comes up. We seem to have agreed by osmosis that Katie is staying with us for the next six months.

As time goes on, I notice two things happen. Heather starts to work later and later in the evenings. They’ve signed a new author and Heather has been given the lead role on the marketing, so there is a lot of planning, preparation and additional stress. At the same time, we start to have sex less and less.

It comes to a head one evening when, as usual, Katie and I are messing around in the kitchen supposedly doing the dishes. We held off on serving dinner for as long as possible, but eventually ate without Heather. We’ve drunk more of the wine than usual. I may be a little bit tipsy and Katie probably is too. We’re dancing to a song playing on the radio and having a tug of war with the tea towel when Heather comes in. She looks at the two of us and just dumps her bag and heads off to the bedroom.

I go after her and find her sitting on the bed. Slipping in beside her, I wrap my arm around her shoulder and pull her closer. She sighs and whispers, “I’m sorry.”

I tilt her chin up and kiss her. My tongue teases her bottom lip as my fingers stroke her hip. I want her and my kisses become more insistent as the bulge in my trousers makes itself known. I guide Heather back onto the bed, leaning over her as my fingers trace the swell of her breast through her blouse.

My tongue flickers over her mouth, probing, pushing its way past her teeth. My fingers grapple with the buttons: one, two, three undone, the lace of the bra cup visible. The little gold crucifix nestles in the hollow of her neck.

I kiss my way down her throat, my breath causing goosebumps to appear on her flesh as the blouse is unbuttoned to her waist. My fingers scoop her left breast from the lacy cup, the pale pink nipple hardening already.

As I blow my breath over it, watching it quiver, she sighs.

“No. Don’t, Mark. Stop, please.”

I sit up, confused. She pushes me away and reaches down to refasten the buttons.

“Not now. Please.”

She heads to the bathroom while I sit on the bed, stunned. She’s never refused to have sex before. I listen to the running water through the bathroom door. Imagining her naked, running her fingers over her naked body as she massages the shower gel into her skin.

I go to the door, wanting to discuss what’s wrong now. I don’t want to wait, but when I turn the handle, the door is locked. She never locks the door. Some of our best sex is when one of us is in the shower and the other sneaks in and joins them.

I curse under my breath and wait but the shower keeps running. I go back into the living room. Katie brushes back a strand of long blonde hair which had fallen over her face and smiles up at me when I come in.

“I wasn’t really watching anything,” she apologises as she hands me the remote control. I slump into the opposite corner of the sofa from her and the two of us sit staring at the screen as I flick through the channels continuously.

“How can we have two hundred and ninety-seven channels and there not be anything decent to watch on any of them?”

“We could always watch that Father Ted episode for the seventeenth time,” she suggests, as Ted and Dougal sing about their lovely horse. I shrug and keep flicking.

“Is Heather alright?”

I look at Katie.

“Yeah, she seems fine. I think she’s a bit tired maybe. Why?”

“Yeah, you’re probably right. She has been working really hard lately. A lot of late nights in the office. I’m sure her Boss is well impressed with her.”

I realise I’m chewing my fingernail as I think about Heather and her late nights at work. It’s true she’s been putting in a lot of extra hours. They’re preparing for a book launch and I’m sure things are getting stressful as they near the launch date. I reassure myself that’s the reason why she seems off with me. She’s distracted with the launch. Nothing more.

I relax and watch Katie out of the corner of my eye as she plays with a strand of her hair. She brushes it against the end of her nose like it is a paintbrush. She catches me looking and immediately turns scarlet, blushing in embarrassment.

“Oh my god, stop looking at me,” she laughs, and I refuse to obey. Instead, I let my eyes linger on her face, thinking it’s been ages since I’ve seen Heather smile or laugh like that. She throws a cushion at me and tells me to behave.

When Heather joins us, she seems more cheerful. She’s wrapped up in her fluffy white dressing gown. The one she’d liberated from the hotel when we had that dirty weekend at the conference I was attending. Despite it being only Tuesday, she opens another bottle and we lounge around chatting aimlessly until well after eleven. Finally, Katie sets a good example and heads off to bed. Heather stands up and pulls me off the sofa and into the bedroom.

In bed, we lie facing each other. She drags her finger along my cheek.

“I’m sorry about earlier. Work is just a bit… intense, I guess.”

I lean in to kiss her.

“Just hold me, please?”

I wrap my arm around her as she turns her back on me and we lie spooning.

I listen to her breathing as it steadies and slips into the regular rhythm as she sleeps. But I can’t sleep. Thoughts begin gnawing at my head. Has she gone off me? Katie has also noticed how many late nights she’s been doing. My brain races ahead of itself. What if she’s having an affair? I’ve met her Boss a couple of times at work events. He was nice, but yeah, he was also attractive and in a position of power and authority over her, and Heather likes to please everyone in everything she does.

Sleep refuses to come. I spend the whole night imagining Heather and her Boss. Imagining how they really spend the evenings when she is working late. Him walking into Heather’s office and just fucking her across her desk. I can see her skirt pushed up around her waist as he takes her from behind, or her dropping to her knees under his desk, giving him the puppy dog eyes as she slips his cock into her waiting mouth.

My hand encircles my cock and I wank as I imagine my girlfriend being fucked. I start to see her with other men: clients, colleagues, the new author she’s working with. I bet he has a big cock too. Maybe she fucks him and her Boss at the same time. Before I know it, my cock is spasming and I’ve splattered cum over her back.

She sleeps on, oblivious as I lie there, embarrassed, wondering what the fuck my brain is doing. In the morning, as she slips out of bed, a fresh rush of guilt floods through me when I notice the semen stains on her nightie.

“I meant to tell you.”

I look up as she pokes her head out of the bathroom.

“I have to go away for a few days. There’s a book fair in Hamburg that John wants me to go to. I’ll be going with Alex. He’s doing a couple of book readings and John wants me to do the marketing on-site.”

When are you going?” A cold hand seems to grip my stomach as I ask.

“Monday, and back on Friday. I’ll still be back in plenty of time to get down to Mum’s for her birthday party on Saturday.”

Her head disappears and I hear the shower turn on as the news settles in. Four nights away. Four nights away with that author, and probably her boss too. Four nights of her being fucked senseless in German hotel rooms.

The thought of her cheating gnaws away at me all day. I can’t concentrate. I see her spread over a German publisher's desk as her boss offers her as part of the deal to publish the book.

I decide to stay out after work and head to the pub with a couple of friends. It feels good to be free for a few hours. I realise having Katie around is making the flat seem claustrophobic. I resolve to talk to Heather when I get home. Maybe we can spend the weekend together and have some fun. Perhaps it’s time for Katie to find somewhere else to live.

When I get back, the flat is in darkness, but Heather’s coat is on the rack so I know she’s home. I slip into bed beside her and she sleepily enquires if I had fun.

“That’s nice,” she murmurs, when I tell her about the evening, and suggests I could maybe take Katie out at the weekend while she’s away.

“I thought you were going on Monday?”

“Change of plan. They want me to fly to Amsterdam tomorrow and then go from there to Hamburg on Monday. It makes more sense than flying backwards and forwards.”

I don’t know what to say so take my usual approach and say nothing.

“Anyway, I need to sleep, so we’ll talk in the morning before I go.”

The morning is chaos. Heather rushes around throwing things into her case. She runs between our room and Katie’s searching for clothes and makeup and we hardly get a moment to talk.

“I’ll be back next Friday. You’ll hardly notice I’m gone.”

I nod disconsolately. Over a week without her. She turns and gives Katie a hug. I can hear her telling Katie to look after me before she turns back and wraps her arms around me.

“Don’t drink too much and have fun... and look after Katie.”

We share a quick kiss and then she is gone. I glance at Katie who is throwing the last of her coffee down the sink and grabbing her coat. I sigh and put my coat on too.

“See you tonight.” She smiles and is gone too.

When I get home from work, Katie is sitting with her feet curled under her like a cat on the sofa. The word that springs into my mind when I see her is country chic. She has on a navy blue jumper and a blue skirt with little yellow flowers on it and black stockings and the whole outfit does nothing but emphasise her flirtatious personality. I slump down beside her. We sit watching tv in silence.

“Do you want to go for a pint?”

Confused, I turn and look at her.

“What? Us two?”

She looks around the room and peeks under a cushion.

“Well yeah. I don’t see anyone else here.“

I nod, realising it might do me good to get out of the house and just let off some steam. We head to the local just down the road.

It’s Friday night and the pub is pretty full. As we order our drinks, a pint of IPA and a bottle of Corona, we realise there’s a pub quiz on. It’s too late to join in, they are already on round three, but we try to answer anyway and I quickly discover Katie is as competitive as her sister.

The time flies and I realise I haven’t thought about Heather cheating on me for hours when Katie looks at her watch and sees it’s almost midnight and we should head home.

“I always thought you were the rebellious type, but look at you. Scurrying home even though tomorrow’s Saturday.“

Katie bristles.

“I can be rebellious if I want to. I just don’t think staying for one more beer is that big a thing to rebel over.”

I shrug and smile, seeing the same brooding expression Heather pulls when she is pretending to be cross too. Without realising I’m doing it, I reach out and tickle Katie the way I tickle Heather. She squeals and squirms, trying to wriggle her way out of my grasp when my hands brush against her boobs. I feel her pointed nipples and immediately let go. The realisation that I’d just grabbed Katie and not Heather hits me.

“I’m so sorry,” I apologise, but Katie just shrugs it off, telling me it’s no big deal.

We head back to the flat in silence. As soon as we get inside, Katie says “Goodnight” and heads to her room.

I go into our bedroom and lie on the bed. The detritus from Heather's hurried packing is scattered around the room. I realise that every time I think of Heather, Katie’s face appears instead. I cringe as I remember grabbing her boobs and her reaction when we got back to the flat. I’m worried I’ve fucked up with Katie and she thinks I’m a pervert trying to get off with my girlfriend’s sister.

And then that voice starts up inside my head. The same one that’s been going on about Heather and her boss and the author and the German publisher all fucking her in a hotel room is now telling me how good Katie looks. How her hair smells, how her eyes sparkle and how she’s even better looking than Heather. And how you just know she’ll do all the things Heather doesn’t like doing. Without thinking about it, my fingers close around my cock. I close my eyes and imagine undressing Katie. How when I tug up her jumper, she’s wearing one of the little cotton bra and pantie sets I’ve seen drying around the flat, draped over the heaters. I imagine she’s wearing the pale blue one with the little bow in the centre. I know her boobs aren’t as big as Heather’s. I checked the size on a bra once. A 32B. Still plenty for her size. I wonder if she’s got the same pale pink nipples as her sister. If she’ll squeal and squirm when I suck the nubs between my teeth. The memory of that snatched moment when I felt her hard nipples through her top as I tickled her makes me squirm. When I think about her sinking to her knees and sucking my cock, the spurts of cum splash over my chest. I lie there, imagining her swallowing every drop as the semen dries on my skin.

I fall asleep with a mixture of emotions, though mainly lust and guilt, vying for attention.