The five of us were masturbating in front of each other. 'This is weird,' Jack kept saying. 'This is really weird.'

'And yet you're still here,' said Samantha. 'Go out the room if you don't like it, or go home.'

Jack and Samantha were siblings. Less than an hour earlier, she'd caught us all playing together. Our sessions always began with solo masturbation and trying to hold ourselves on the edge of orgasm. But by the time she'd walked in unexpectedly, we were well into touching and pleasuring each other.

Fair play to Samantha for joining us instead of freaking out. I don't know what I would have done if I'd caught my little brother doing anything sexual – and I certainly wouldn't want us to orgasm in front of each other.

Now she’d joined us, we'd gone back to the solo stage and were holding off again. It had a different feeling for us girls this time, though, and a different sound. Samantha had cleaned her vibrators and let us borrow them. Because Tiff and I don’t masturbate in the same way, we found two different toys that suit our particular styles. I chose a small lipstick-shaped one that was perfect for reaching the sweet spot on my clit hood; Tiff chose an insertable that was twice as long as her usual brush handle.

The two toys almost harmonised with each other, with the higher and steadier vibration from mine alongside the lower tone from hers. The sound changed slightly from time to time as we adjusted the position for a better feeling. I can't tell you what a step up it was from just fingers; I frequently had to lift it away to stop me from orgasming right there.

While we self-pleasured, Samantha asked what Tiff and I had done together. She already knew about the group sessions at school, so we told her about a few of our own activities, culminating in the humping experiment that didn't work out.

'Oh yes, tribbing.'

'Tribbing?' I asked. 'Is that the same thing?'

'Kind of. It doesn't work for most of us, though, especially with innies like yours.'

'Innies?'

'Your clits don't stick out enough. See, I've got an outie and even then, it's still hit-and-miss.' I'd already noticed her size. It was about the same as the tip of her little finger. I felt a little jealous; I wanted an outie too. 'A good method,' she continued, 'is to stick a vibrator between you both and hump that. It's not strictly tribbing, but it usually works far better.'

Samantha also had a few toys that didn't vibrate. At that moment, she was pushing a penis-shaped dildo in and out of herself. The other hand worked her clit, trapping her hood between her index and middle fingers to move it rapidly back and forth over the head without exposing it too much. From touching each other, I knew Jack didn't like his foreskin pulled back too much, preferring quick strokes just over the head. It was remarkable how a sister and brother could share such a similar technique.

'You mentioned Kath and Michael earlier,' said Samantha. 'What happened there?'

We all started to chip in with parts of the story.

When we started as an all-female group, our classmate Kath had opposed us becoming a mixed-gender group because her first cousin Michael was one of the boys. He still joined us, but on condition they sat back-to-back so they wouldn't see each other naked or masturbating. They soon became more comfortable in each other's presence, and would even compete over which of them could orgasm first.

Then recently, someone had seen a message from Kath on Michael's phone saying 'I'll have the house to myself' on a certain date and that they could 'be like a proper couple for one night', followed by lots of kisses, and that had started the gossip.

'That was just a rumour,' said Jack. 'They didn't actually confirm it.'

'I saw the message. There was no two ways about it.' Tiff nodded in agreement.

'Well, whatever floats their boat,' said Samantha. 'It's legal isn't it?'

'It's still weird,' said Jack. 'A bit like this.'

'Oh, either shut up or don't look.'

It wasn't like Tiff to be nervous about saying anything to anybody, but for the previous few minutes, she'd looked like she was about to speak, then stopped herself.

'Samantha? Can we try it?' she eventually said. 'This tribbing thing? Like, with you.'

She broke into a smile. 'Of course we can.'

'Eww, I'm not watching that. That's weird.'

'Then you two go and leave the ladies to have our fun.'

I really enjoyed when she referred to us women or ladies instead of girls. Between that and using a vibrator, not to mention my taste for semen, I'd never felt so grown up. I supposed I should also start saying 'men' for the two guys, although still I thought 'boys' suited them better.

'I want to watch,' said Steven.

'Of course you can,' said Samantha, 'it turns you on, doesn't it?'

There was no specific time when the solo part of the evening would switch to pairings: often the flirty talk would build to a head, sometimes one of us couldn't stop ourselves from orgasming, and occasionally it just went on too long.

Samantha sat next to Tiff, their toys still inside them. 'Do you want to finish off together before we trib?' Tiff nodded, and increased the speed of her vibrator. As well as masturbating in our own ways, Tiff and I were also different when we orgasmed. I could do it almost silently, but Tiff was a screamer. She'd had to stifle it in the school changing room, but here she could let rip. Samantha turned out to be different again, grunting, 'Uh, uh, uh,' slowly and increasingly loudly as she got off.

After their orgasms, they turned to face each other. Samantha removed her dildo then took the vibrator Tiff was using and placed it vertically between them. As they shuffled for a few moments to let it hit them in just the right way, I spotted the boys had started to touch, and their erections rested on each other's stomachs. Jack was trying not to look at Samantha but Steven didn't show any such restraint.

Which left me, the odd one out. All I could do was play with myself while watching the others and wonder when and how I would get to join in. Both pairs were too focused on each other.

I finally made an in-road when Jack spurted all over his stomach under Steven's control. Knowing I loved the taste, they always let me have it, so I was surprised to see him licking it off Jack.

'Oh,' I said, 'I didn't realise you swallowed.'

‘Sorry, I got carried away. I should have left it for you.’ I didn’t miss the chance to cream off the rest while it was still fresh.

'I don't swallow anything,' added Jack. ‘Only he does that.’

I heard his sister starting to orgasm again as her grunts grew increasingly loud. Tiff reached over to grab the dildo that Samantha had used, explaining she needed something inside her. They pulled back briefly from each other as she inserted it, then moved back together, placing the vibrator between them once again.

I wanted to play some more, so I reached out to Steven and began to stroke his foreskin. But Jack was now the one who felt left out. As I tried to get into a rhythm, Jack got in my way, trying to touch and kiss him. Steven, in turn, was still fascinated by Tiff and Samantha, who kept kissing each other and having orgasm after orgasm.

Somehow avoiding Jack's clinginess, Steven reached over and started stimulating me. It felt nice because it was the first time I'd been touched that day by anyone except myself, though despite my best instructions, I knew I wouldn't orgasm from it. Maybe it's because boys are used to big hand motions, whereas we need little finger touches. However, I'd become an expert at those big motions and it didn't take long before he shot over my hand. Now I knew he liked it, I offered that hand to Steven.

'Eww, no.'

'I thought you swallowed.'

'Not my own, only his.'

'Oh, I see,' I said, licking it up myself. ‘I thought you would. I would. I always lick my fingers afterwards.’

The tribbing took some time to finish, and they were exhausted from it. Tiff’s hair was pasted to her forehead by sweat. 'All right,' said Samantha, grabbing her laptop, 'it’s bedtime. Women in my room, and the boyfriends can do what they like.’

‘We’re not a couple,’ Jack always said reflexively.

‘As you wish. Goodnight.’

As Samantha led us through to her bedroom, I was curious. 'Can I ask you about... something... about your clit?'

'Ask me what you like.'

'Do you not get turned on all the time? Does it not rub against your panties?'

'It does a bit, especially with a pantyliner, but I'm used to it.'

'And can you... you know... put it into...?'

She nodded and smiled, knowing where I was going. 'Penetrate a woman? Believe me, I’d love to and I've tried, but it just doesn't reach.'

'I see.'

'Now,' she opened her laptop, 'you ladies said you wanted your own vibrators, right?' Tiff and I smiled. 'So let's order some.'

I looked forward to that, but I also knew our group wouldn't work with five people, especially as four of them naturally paired off with each other. I needed another boy, and I carefully considered my options.