For about a year, I'd been part of a mixed-sex masturbation group with my classmates, meeting up every Friday to pleasure ourselves and – more recently – to have sex with each other.

But for the first time, I had to miss a week because my parents were taking my brother and me on our annual holiday. They always timed it around the time of our birthdays. This year, Ryan had turned sixteen about two weeks before I turned eighteen. We were going to spend seven days in a static caravan where I'd need to share a cramped bedroom with him.

Even on the days the group didn't meet, I was still giving myself at least one orgasm a day, so I'd need to be sneaky. What's more, my period was fast approaching; I'd always feel horny in the days leading up to it and masturbation helped take the edge off any cramps. Fortunately, I'd shared with Ryan on many holidays before; I would just wait until shower time. I could probably even squeeze in another one while he was washing.

Unfortunately, there was a snag in that plan. The caravan was fitted with a tiny shower, but the water wouldn't heat up. My parents called reception, who sent a plumber, and they spent an hour trying to fix it before giving up. There wasn't another available caravan of that size, and the replacement part needed for the shower wasn't available, so we would need to use the communal showers.

We all went down to the showers as a family. There were two blocks, one for each gender, and separate cubicles within each block. I tried to pleasure myself but I couldn't finish, knowing my mum was only a few feet away and other women were coming and going.

Then all day we were going for walks, and looking round castles, and eating in restaurants, and basically spending every minute together as a family. At night, my bed was barely a foot away from Ryan, so I could touch myself but not with nearly enough speed or friction for pleasure or he would notice. I even tried doing it in the caravan bathroom and any public toilets we stopped at, but without the noise of a shower drowning out my breathing, I didn't want to risk being overheard. I couldn't even sneak out to the shower blocks because they were locked at night.

After three days, it was affecting my sleep and my cramps were getting worse. I was putting on my night-dress in the bathroom and I realised I absolutely had to get off or I wouldn't be able to get any rest. But how do you bring that up with your brother?

Back in the bedroom, I lay down underneath the covers. Ryan was in the other bed on top of the blankets playing a computer game – in fact, that game never left his hands most of the time. He was only wearing shorts.

'Ryan,' I said, 'I need to...' He didn't look up from the game but raised his eyebrows. I was hoping to be able to think of the right words as I went along, but it wasn't happening.

'You need to what?'

I took a deep breath, thinking of a slightly different angle. 'You know how boys all wank? And don't say you don't because you all do.'

He paused his game and looked at me but didn't say anything. He looked a little disgusted, though I wasn't surprised.

'Well, so do girls and there isn't a moment of peace to do it here or the showers, so I need to...' I closed my eyes as I said it, 'I need to... Touch myself. Like, tonight; like, now. I'll do it under the blanket so you don't have to see anything, but I'm not going to be able to sleep because I'm so...' I didn't particularly want to say it, 'Horny.'

He didn't seem to know what to say at first. 'You need to... Touch yourself? You mean, like...'

I nodded. 'Yes, down there,' I said, sparing him from having to end the sentence. 'You must have to do it as well?'

He stared for a bit. 'Could you not do it in the bathroom?'

'I've tried but I can't get comfortable and,' I pointed to our parents' room, 'they'll hear me. Look, I'll do it under the blanket,' I repeated, 'you don't have to see anything.'

He turned to face away from me. 'Just say when you're finished.' He put on his headphones and went back to his game.

'You're the best.' I hadn't brought my vibrator because I thought I'd be doing it in the shower, but I let my hand slip down to my clit straight away.

I only took a couple of minutes to get off, immediately releasing three days of frustration and partly relieving my cramps. I knew from the start I was probably going to have at least two, so I began again almost as soon as the first one subsided.

I was so lost in my thoughts that I barely even registered my brother in the same room. I was glad he took it so well; I was half-expecting him to freak out. I looked over at him as I continued to play. It was difficult to tell with him facing away from me, but it looked like he was squeezing an erection through his shorts. I was right: he must have needed to orgasm as much as I did. I decided to give him the green light.

The room was so cramped that I was able to keep one hand on my clit and reach over to him with the other. I lifted off one of his headphones speakers and whispered, 'You know you can have your fun as well, if you want to.' Hearing my voice, he immediately stopped his squeezing and pulled his hand away. 'Have you managed to get off at all since we've been here?'

He hesitated. 'Just once.'

'I'll turn the other way and finish off. You say when you're finished.' It's a bit harder for me to masturbate on my side, but I was so horny, I knew it wouldn't make much of a difference.

I heard him go under the blanket and take off his shorts, then there was the faint rhythmic sound of skin I'd heard so many times from the boys of my masturbation group. I didn't mean to listen so intently, but I found it fascinating.

It didn't take long before I heard my brother's rhythm slow down and his breathing become a little heavier, so I knew he was at the clean-up stage.

'Have you got tissues?' I asked.

'It's okay,' he said.

'Right. I'll have one more then I'll stop,' I told him as I drove towards my third orgasm. It turned out to be almost as good as the first.

Once we'd washed our hands and dressed for bed again, I told him, 'Any time we need to do that, we just do it, all right?'

Ryan nodded. All he wanted to do was go to sleep instead of being asked about his sex life by his sister, but I was curious. Since we'd already broken the big taboo, a few questions seemed tame in comparison.

It took about half an hour for him to answer, during which time I told him about my experiences of masturbating with my classmates and the boys I'd been with. He talked too about his girlfriend. Marnie was a year older than him and had already had sex. Ryan, on the other hand, was still a virgin yet she'd seemed reluctant to go all the way with him.

Over the next couple of days, a barrier had definitely been broken down. It seemed we had similar sex drives, which I suppose was part of being siblings. Each of us liked an orgasm before getting up in the morning and after going to bed at night.

Most of the time, we kind of acted like the other person wasn't in the room until we were each finished. However, he did confide in me that he'd never seen female masturbation in real life so we watched each other a couple of times. Interestingly, he rubbed his semen into his chest and stomach instead of cleaning up with a tissue.

Then on the second-last night of the holiday, we found ourselves talking about going down on boyfriends and girlfriends. He told me that he and Marnie had recently gone down on each other, but it hadn't worked for either of them. I said I'd had a lot of good experiences in that area, which I'd perfected on a few different boys.

The discussion ended up with me taking the head of his penis into my mouth. I made it clear I would only give a few strokes to demonstrate my technique and give him the sensation of what good oral should be like. Those few strokes turned into lots. I'd even felt the twitches that meant he was about to spurt, yet he showed no sign of wanting me to stop, and I didn't want to stop either. I placed my tongue into position to avoid gagging as his semen flowed into my mouth.

Without thinking about the person it had come from, I swallowed it all and lifted my mouth away.

We were quiet for a few seconds. 'Well,' I said eventually, 'I think it goes without saying that shouldn't have happened.' Still, a little more fluid ran out of him and I absentmindedly scooped it up with my fingers and licked them. 'Agreed?'

'That felt really good.'

I climbed off his bed and onto mine. 'I know, I loved it, but we can't. I'm going to finish myself off then go to sleep.' By this stage, I didn't even need to hide under the blanket.

'Can we not do it again some other time? I liked seeing you swallow it.'

Beginning to stroke my clit, I said, 'Don't get the wrong idea; it's just not right with siblings. Maybe you could ask Marnie to try it again. I'll talk you through my technique again.'

'Yeah,' he said, sounding defeated. 'I'll ask.'

I had a thought, although I knew full well it was my horniness talking. 'Okay. If you liked seeing me swallow, maybe... Maybe we could compromise. I know it's not the same, but maybe you could just give me your spunk?'

'Give you it?'

'Like, catch it in your hand and I'll take it from there.'

'So you can swallow it?'

'Just for the rest of the holiday. So you get to see me do it but we aren't touching each other's parts.'

'That's amazing.'

'It's a deal, then.' I hadn't stopped masturbating, but I closed my eyes at that point and acted like he wasn't in the room.

It was an arrangement that worked for the rest of the holiday, yet we still ended up talking sexually and occasionally touching each other. We dry humped at one point; I kept my underwear on to stop accidental penetration. I even gave him oral again the day before we went home.

We developed a kind of unspoken rule that we didn't look directly at each other when we made physical contact and kept our words functional, never romantic or flirty.

When the holiday was over, I imagined we'd put this period behind us and go back to being like any other sister and brother. Yet some sort of dam had been broken and we felt able to talk a little more about our sex lives and what experiences we were having, and that led to us becoming closer.