While on holiday with our parents, I needed to masturbate badly, but there was no privacy. I ended up asking my little brother if I could do it while he was in the room. This had evolved into other sexual contact up to and including oral – but not penetration.

Once we were home, I expected us to put this strange period behind us and go back to being like any other sister and brother. Yet we'd broken through a barrier and we occasionally still ended up together. Usually it was watching each other masturbate, but a few times, he came to my room to hand in semen when he'd ejaculated alone. He once secretly gave me some when I had a boyfriend over, so I swallowed two loads within ten minutes.

Ryan had been with his girlfriend for a few months and told me they were planning to have sex. He was a virgin, he told me, but she wasn't. As such, I'd helped him to prepare by showing him how to put on a condom, and we'd dry-hump with underwear on so he could safely feel what different positions were like.

On the big night, just as he was showered and about to dress, she'd called him and not only backed out of it, but dumped him. According to Ryan's account, she tried to justify it with some flimsy excuse about not being ready, but we both knew that was a lie.

'Ryan, I know it's hard,' I said, 'but forget about Marnie. You'll meet other women, one who's lucky to have you.'

'Why did she have to say we'd have sex then just – bam – that's it finished?'

'I don't know. It's not right.'

'It's not fair. She just dumped me.'

We talked about this for a while, the conversation beginning to go in circles. I reckoned he needed some release. I knew he'd avoided playing with himself too much ahead of the occasion.

'Ryan,' I said, running my hand down to his shorts, 'do you want to?' He gave a little enthusiastic nod. 'Let me shut the door.'

On the few occasions we touched each other, we had an agreement. I wanted him to be good at making a woman orgasm with his fingers, something my boyfriends were always hopeless at doing. So the deal was that he would have to get me off first, and then I would return the favour.

I considered letting him off under the circumstances, but I decided against it. I stood up and lifted my arms, letting him undress me, just as we'd practiced many times: top, jeans, then underwear. At that point, we hardly ever kissed on the lips, only elsewhere on the body. There was no rule against lips, but it simply wasn't our habit.

Once I was naked, I lay on the bed next to him as his fingers found their spot on my clit. He needed only a little instruction by this time, and it didn't take long before I felt the sensation begin to build up; it helped that I hadn't already masturbated that day.

After I orgasmed, we both licked Ryan's fingers. I put my panties back on as a barrier as his sperm sometimes flew everywhere. I then looked briefly at the clock before starting to undress him. I thought how he would probably have been well on his way to entering his first vagina by then, but he now had to make do with his big sister's hand.

I found myself thinking Well, he could use my vagina, but I tried to dismiss the words as quickly as they came. Even though what we were doing was hardly appropriate, that pushed it to a new level.

But I kept thinking about the possibility as I stroked his foreskin up and down. My brother believed he had it on a plate: he was a sixteen-year-old virgin about to pop his cherry with an experienced seventeen-year-old girl – at least by her own account. That's the stuff of teenage fantasises.

I inadvertently slowed my strokes right down, which caught his attention.

'A bit faster,' he said, 'I'm nearly there.'

'Ryan,' I said. He raised his eyebrows. Just like that third night in the caravan, I was hoping I'd be able to think of the right words as I went along. 'I'll... I'd like... I want to give you an option. I won't... it doesn't matter what... it's a free choice, all right?'

'What option?'

'I can either finish you off right here like normal, or... you know how... you know the condom you had for tonight? We could... um...,' the words were becoming harder, 'use that instead. Up to you. Doesn't matter what you choose,' I added, badly trying to play it cool, 'doesn't matter to me. I won't ask again.'

He looked thoughtful. 'Are you asking what I... think you're asking?'

'Just what you would have been doing tonight with Marnie, but with me instead. It won't be the same – nothing romantic,' I lowered my voice when I remembered our parents were in the house, 'just... functional, just a release.'

'Right.'

'And no kissing, and only from behind, so we don't look at each other.'

He looked thoughtful again, then gave an enthusiastic nod. I felt a weird sense of relief. If he'd said no, I didn't want any tension between us.

'Just like we practiced, then.' I took off my panties again. He found his condom and put on it just as I'd shown him. I was horny, but I still masturbated a little to help my wetness flow a bit more.

I went on my hands and knees, then balanced on one hand to open my vagina with the other. This was where I would find out what my little brother was like in bed.

I felt him finding the entrance with his penis and resting it there. Once it was in position, I removed my hand, expecting his first thrust, but he seemed to hesitate.

'It's just a release,' I repeated, 'and don't try to last, just finish as quick as you can.' I knew I wouldn't have time to orgasm, but I'd already had my fun. This session was about him now.

But that thrust still didn't happen.

'Are you not going in?' I asked.

'Yeah, in a minute.'

'I don't have a minute,' I said, knowing that was his way of giving an indefinite 'no'. I pushed my hips backwards and made him enter me. He didn't try to pull back. He let out a gasp as he slipped into his sister. 'And don't hold off, just think about Marnie or something.'

So this was incest? I found it just like being with any other boy, except I was trying not to think about the person I was with. From our humping sessions, we'd both become good at detachment and looking away from each other, even when we were face-to-face. I imagined I was with one of the boys from my masturbation group, then included other males in the fantasy. Ryan's penis was just there for the sensation and he would no doubt be thinking about women he wasn't related to.

It could only have taken two or three minutes before he ejaculated safely inside me.

'Okay, clean-up,' I said. 'Remember what we do?'

'Pull right out, then take off the condom.'

'Correct. Right out and away from the woman. We want to contain the spunk.'

I lay on my back again and gently masturbated, enjoying the sight as he pulled off the condom full of white liquid and tied a knot in it. Post-ejaculation was also one of the few times I saw Ryan naked without an erection and it was so cute.

I'd just had full penetrative sex with my own little brother, yet it didn't feel as though anything wrong had happened. In fact, he asked if we could do the same again at some point and I found myself saying of course we could, but it would be done sparingly. I made it clear our default sexual activity would still be masturbating together, and he was pleased enough with that.

It also went without saying that we would tell nobody. We were close with our older sister, who was six years older than me. Rachel was pretty open-minded, but we agreed this was just too much, so it would be strictly between us.

At around the time we broke the penetration boundary, I was about to go to college in another town, so we wouldn't be together so often. In a way, that made it more special, as we would only manage to spend one night together every few weekends.

Any time he was over, he would always leave a sleeping bag on the floor. I shared a flat with other students, so that was there in case the door was open and anybody happened to look in. Secretly, he would sleep in my bed and we would stay as quiet as possible while we had our fun. I found it interesting that he was happy for us to refer to each other as 'siblings with benefits', but only when we weren't doing anything together. Weirdly, if I mentioned words like 'brother', 'sister' or 'sibling' during our sexual activity, he would start to lose his erection.

I should have suspected it from Marnie's age, but it seemed Ryan had a liking for older ladies. Once, we were in the student union and he wouldn't stop flirting with my nineteen-year-old classmate Fiona. Even though he was seventeen, and I was there, and the bouncer had drawn a black 'X' on his hand so he wouldn't be served alcohol, she kept returning his advances. At the end of the night, they went back to her place.

I was secretly excited for him and I wanted to find out whether he'd put my teaching to good use. I made him tell me all about it the next morning, and it sounded like he had. I'd wanted to hear Fiona's side of the story too, but for obvious reasons, I didn't ask. I heard they slept together a number of times after that.

Although our default activity did remain masturbating together, our sibling benefits did evolve in two other directions.

Firstly, we would increasingly kiss on the lips. It initially started happening before we undressed each other, then after penetrative sex – but not where I'd swallowed his semen, as he didn't want to taste it. Eventually, we did it casually when nobody else was around.

Of course, we were nearly caught on a couple of occasions. I swear one of my flatmates spotted us. I don't know for sure, because he never said anything, but he did start acting weird around me, ending all our conversations as fast as they began.

The second direction was my initiative and was much more exciting. Rather than tell him outright, I decided to set it up as a surprise.

I told Ryan I thought it would be great if we could act like a proper couple for one night. Just after he turned eighteen, I booked a train to a city miles away where nobody was likely to know us, and found a cheap hotel room.

He was nervous about holding hands at first in case anybody saw how similar we looked, but I assured him that people would never make that assumption. I'd made us a dinner reservation, then we moved on to a bar. By the time we left there and headed to a nightclub, he had no problem showing affection. I told him to mind his alcohol intake, though.

We danced much of the night with our hands all over each other, liberally kissing, often with tongues, not caring who saw us because they all thought we were girlfriend and boyfriend, not sister and brother. At the end of the night, we headed to the takeaway. While we were eating, I mentioned there was a reason I told him to limit his drinking.

'It's a threesome, isn't it?' he immediately asked.

'How predictable.'

'You, me and Fiona would be nice.'

'Well, it's not.'

'Who, then?' He paused. 'Wait... it's not with Rachel? Is it?'

'Would you do it with her?' I asked.

'Um... yes, maybe?'

'It's not a threesome at all. Not unless you know a bisexual guy, then maybe.'

'Why? No, it's got to be a girl.'

'Anyway, it's not a threesome, but it's something we can both enjoy.'

'What is it?'

I told him I'd spoken to my doctor and moved from the pill onto a contraceptive implant so I could never forget to take it. As a result, I'd cautiously stopped using condoms with boyfriends. In about six months, we'd never had a scare, so I was sure I could rely on it. I enjoyed delivering the final line I'd been rehearsing for so long:

'What I'm saying, Ryan, is that I'd like you to spunk inside me.'

He gasped a little and I could just tell he was immediately hard. 'Oh wow. I've never done that before.'

'Now, that doesn't mean we'll be having sex more often, but I think tonight would be a good start.'

Unusually for us, that was one of a handful of occasions where we'd done it face-to-face. The other times, it had simply been the most comfortable way at that time, but it happened for no reason that day.

Once he'd slipped in, we looked away from each other and thought about other people. I loved that there was a little more friction without a condom, which made him finish quickly, and I loved knowing that he'd sprayed all his semen inside me.

There seemed to be even more of the stuff than usual. I cupped my vagina as I sat up and sure enough, it seemed like twice as much ran out. I swallowed it, of course, just as I did with other boys.

I cheekily asked, 'How do you feel doing your big sister bareback?' He didn't answer me, but he lost his erection quicker than usual.

And that's how our relationship developed through college and afterwards. Although watching each other masturbate remains our main sexual activity, all it takes is a discreet look any time we meet, and we go and find a private place.

I love my sibling with benefits and I hope we'll always have our secret fun together.