College had ended for Christmas and I looked forward to going back to my parents' house to see my siblings. Ryan was younger than me by two years, and we'd met up during term-time but not very often; Rachel was six years older than me and I saw her even less.

Ryan and I had started a top-secret relationship, stemming from a caravan holiday. There had been no privacy to masturbate so we agreed to do it in the same room and had progressed to watching each other. We mostly still stuck to that activity, but we now included oral and vaginal sex from time to time. These were simply an outlet for our sexual energies and nothing to do with affection. Penetration was almost always done from behind to avoid eye contact, and we would think about other people. 

Our parents' house would usually be occupied at any given time, and Rachel and I were sharing a bedroom like we used to, so she would be around a lot. My own orgasms weren't affected because I could do that in the shower, while Ryan had his own room, but it was inevitable that he and I would make the most of our time together. The question was when we would manage it.

The opportunity arose a few days before Christmas when Rachel managed to find a last-minute hair appointment. Separately, our mum had gone to pick up the Christmas food. I'd never understood why we needed so much for one day, but it meant she was out the way for a while.

The moment I saw our dad go into the garage to tinker with the car, I went to Ryan's room and suggested we make the most of our limited privacy. Even before I asked, I could already see the outline of his erection through his shorts. I closed the door and we quickly kissed with tongues for the first time in weeks.

When we broke, I pulled down my jeans and underwear and sat on a chair while he opened the zip in his shorts. We then watched each other furiously masturbating, both of us unashamedly enjoying the sight of each other's genitals. I hadn't orgasmed that day, so I knew I wouldn't take long. But for Ryan, like all boys, he only had one shot, so he would want to last as long as possible.

Although I'm female, I'd learnt how to ejaculate every time I orgasmed. I would stimulate just the correct spot inside me and a little clear liquid would shoot out. As I wiped it off the chair with my hand, I asked quietly, 'Do you want sucked, then we'll take it from there?' He nodded eagerly. 

Whenever we indulged in oral, our agreement was that Ryan would go down on me first then we would swap over. He kneeled in front of me as I took off my T-shirt and unhooked my bra. Running his tongue slowly up and down my front, he found my clit and focussed his efforts there. Over many sessions, I'd taught him just where I feel the most pleasure, and exactly where to place his fingers inside me and what pressure to use.

I played with my nipples as he settled into a rhythm. I often wouldn't orgasm from his tongue alone, but he was good at it and becoming better all the time, so his efforts did sometimes succeed. After ten minutes, I was so close. I told my brother to push his fingers a tiny bit deeper with a lot more upward pressure, and this slight change in movement made me feel like I might orgasm a second time. He was keeping himself hard with his other hand, ready for me to go down on him afterwards.

We badly misjudged how long we would have alone. We heard a quick knock at the door before Rachel let herself into the room. She'd already started asking, 'Ryan, have you seen my...'

Ryan pulled away as she spoke and we both tried to cover up, but she'd seen everything.

'What on Earth are you doing?'

We didn't say anything. 'You're not going to tell mum and dad, are you?' asked Ryan in a small voice.

'Of course not. Ew, I just want to forget... this... what you were doing.'

But despite her disgust, she kept asking questions: how long we'd been doing it, what other things we did, how it all started, and so on. We'd already been caught, so there seemed no point in denying it. I told her the minimum to answer the questions, as did Ryan, but we barely had time to speak before she asked the next one, and she kept telling us, 'You need to stop this, you need to stop.'

She had further questions for me when I was trying to sleep that night, like whether Ryan and I were also seeing other people, what contraception we used, even what sex position we preferred, and so on.

There were large parts I missed out of the story. I told her I had a boyfriend when I actually had two, and they were also in a relationship with each other. I even shared them with my flatmate Fiona, although us ladies were strictly friends and not girlfriends. I also didn't feel the need to mention I knew the boys from a masturbation group we'd formed at school.

What I did say is that I relied on my implant instead of condoms. But what shocked Rachel most is when I said Ryan didn't have a steady girlfriend and had slept with Fiona a few times, with my full knowledge.

'How could you let him do that?' she asked.

'He's old enough. He's not a little boy any more.'

Over the next few days, my sister didn't ask quite as many questions, but still kept telling us both we had to stop what we were doing. My brother and I didn't dare do anything further – even masturbating together – in case we were caught again, but we'd done too much just to stop it completely.

Our parents are not big drinkers, but on Christmas Day, we always shared some wine. Ryan was now 18 and could join us in the tradition for the first time. Rachel, however, drank substantially more than anyone else.

The moment she could have a quiet word with Ryan and me, she said to him, 'I want to know what it's like.' I overheard this.

'You want to know what?' he asked.

 She responded by grabbing his arm and leading him to our bedroom. She closed the door behind us and began to remove her jeans and underwear, leaving her top on. 'What you were doing the other day with her. If it's so good, I want you to show me.' She opened her vagina with her fingers a little to show her clit.

'But you're... you've got a boyfriend.'

'She's got a boyfriend as well, and she's also your sister, so you can't use that excuse on me.'

Ryan looked at me as I sat down on my own bed. She'd promised not to tell our parents, but I didn't want to risk her breaking the promise. 'Go on,' I told him, 'do what you do with me. Unless she tells you differently, of course.'

When he turned back to face her, Rachel was lying against the headboard of her bed, gently stroking her clit. Ryan climbed onto the bed and she again opened her vagina. He placed his mouth over her clit, found his spot and began working on it. He then slipped his fingers into her, just as I like, but she told him, 'Nothing inside, just outside.'

He removed his hand and used it to take his erection out of his shorts. There was still a little of her fluid on his fingers. He rubbed the fluid around the purple head before beginning to pull his foreskin up and down, all the time keeping up his rhythm on her clit.

The sight of him masturbating turned me on, and I slipped my hand into my own panties without taking anything off. Rachel rested her hand on the back of his head and he responded by increasing his speed. She seemed to be receiving great pleasure from his work, squeezing one of her own boobs from outside her bra, and she let out a couple of moans before holding her breath for a few seconds.

I thought she'd got off at that point, but I'd seen a lot of women orgasm and it didn't look like it. She removed her hand from the back of his head. He continued working with his tongue before she pushed him away gently and started rubbing her clit much faster than previously. Ryan and I were both still touching our genitals, and we sped up.

For the first and only time, all three of us masturbated together as a sibling group. We were all close to orgasm, and it happened in the order we'd started our stroking.

Rachel's stimulation only lasted for about a minute and a half until she squeezed one of her boobs even harder and brought herself to a quiet orgasm; our parents were still downstairs. She didn't lick or wipe her fingers before covering up.

Ryan hadn't taken his eyes off her. The sight of her orgasm led to him shoot his semen all over his hand, and it was one of the largest loads I'd seen him produce. This sent me over the edge, making me ejaculate my fluids into my stroking hand.

By habit, I held out my wet hand to Ryan; by habit, he scooped up his semen and gave it to me. Mixing our ejaculations together, I licked it all up, causing Rachel to say, 'Eww, you do that?'

I swallowed. 'Doesn't everyone?'

'Eww, no. I spit it out.' Rachel stood up. 'Right. That was... good, but never again. Just...,' she walked towards the door, 'you need to stop doing all that... stuff, okay?'

She left before we could answer, but she never gave us any more warnings, nor mentioned our sibling fun again.

'Good work, Ryan. I've seen a lot of girls orgasming and I think she liked that.'

'Thanks.'

'We'll keep doing stuff together.' I whispered. 'Before I go back to college, I promise I'll suck you. And just this once, you don't even have to do it to me first.'

I tried to kiss him on the lips, but he turned away as he didn't like to taste his own juices. I kissed him on the cheek instead, and kept my promise on New Year's Day.