My brother had come to visit for the weekend. After our meal, he squeezed onto the small couch between my flatmate and me, with his arm around her. She was subtly flirting back.

Us ladies shared a bedroom, and later that evening, he would be sleeping on an inflatable mattress between our single beds. Ryan and Fiona weren’t a couple, but they’d had sex a few times. I'd already said I didn't mind them doing whatever they wanted, even if I was in the same room; in fact, they knew I’d planned to masturbate that night.

'Why have we never invited you into our group?' she asked, 'with Steven and Thomas and all that?'

None of them knew about our sibling sex life, of course, but it was hard to hide how open we were with each other. 'No,' I said. ‘Ryan’s asked so many times, but it's not right.'

‘It's not like you're doing stuff together,’ said Fiona. ‘You've got Samantha and Jack as well, they're sister and brother.'

'That's exactly my point,’ said Ryan.

'No, just no. It's different here because you know how close we are, but it's hard to... the rest of the group just wouldn't understand that.'

'Of course, they would,' said Fiona, picking up her phone. 'I'm going to ask them.'

'No, you're not.'

She didn't listen to me and instead typed a message into our group chat. I reached for my phone and told them to ignore Fiona’s words, but they ignored me instead.

'So who's in your group?' asked Ryan.

She showed him the phone. 'There's Steven and Thomas, they're our boyfriends, both of them, and they're also boyfriends with each other. Then there's Tiff and Samantha, they're a couple, but Tiff swings both ways.’

‘Wait, I remember Tiff.’ He turned to me. ‘She used to come round and see you all the time.’

‘You wouldn’t recognise her now.’ I had a recent picture to hand, and I showed him my screen.

‘Wow, she’s changed a lot. She’s shaved off all her hair.’

‘And Jack,’ continued Fiona, ‘I've never met. He was put out of the group before I joined it and he's only just been allowed back in.'

'What did he do?'

'Many things,' I said, ‘but mostly, him and Steven used to be a couple, then out of the blue, Jack broke up with him before college,'

'Everyone's in favour so far,' said Fiona. ‘Question from Thomas: are you gay or bisexual?'

'No,' he replied quickly.

Fiona spoke as she typed the reply. 'One... hundred percent... loves the... ladies. And sent. You like them older as well, don't you?'

He didn't know how to answer.

'Cheryl was twenty-two,' I said. 'His ex.'

An in-joke had developed earlier in the evening, where us ladies were calling him Young Man Aged Eighteen to Twenty-Four.

'Really? You're a darker horse than I thought, but still in Young Man Aged Eighteen to Twenty-Four territory.'

'STOP that.'

Fiona read her phone again. 'Thomas is disappointed, but Tiff is very much looking forward to seeing you again. Says she remembers you when you were little, and wants to know how big you are now – in all departments.'

'That's private,' said Ryan. 'Don't tell her that.'

'You'll be getting it out in front of her anyway.’

'Right,' I interrupted, ‘there's no way he's joining us.'

'Well, everyone else is keen. That's Samantha saying yes as well. Only Stephen seems a bit hesitant about new members.’

'Good,' I repeated. 'Tell him I agree.'

Fiona typed as she talked again. 'No... he's lovely... and he has a...' she felt his erection through his clothes, 'substantial... package.'

'Fiona!' we said, almost in unison.

'Is that not the truth? Oh, now Jack's getting annoyed. He thinks what Steven said about new members was aimed at him.'

I grabbed my phone again and said there was no way Ryan was joining the group. The rest of them, except Steven, disagreed immediately.

'It looks like you and Steven are outvoted,' said Fiona. 'Welcome to our group, Young Man Aged Eighteen to Twenty-Four.' She finally put her arm around him and kissed him on the cheek. He didn't even complain about his nickname that time.

'I'm still not happy about this,' I said. Their kiss on the cheek had triggered off kissing on the lips, and they carried on as I spoke. 'We need to talk before anything happens.' I counted out the points on my fingers. ‘How to behave, getting STI tests, where your spunk goes, everything.'

Fiona broke the kiss to take off his top. She then arched her back and turned away from him. ‘You can unhook me.’ He immediately responded, but as if only just realising what she'd said, she turned to me, 'You really don't mind, don't you?'

I shrugged as her boobs were set free. ‘I’ve literally watched you having sex before.'

'With our boyfriends. Not… your brother.'

'Like I always say, we all have needs.'

Even I felt the sexual tension, and I followed in taking off my top. I turned my back to him. 'And me.' In private, he would unhook me in a heartbeat, but he hesitated with company. 'Come on. I'm not asking you to touch me up, just give a girl a hand.' He loosened my bra and I shook it off.

Fiona started kissing Ryan again, and it didn’t take long before she whispered, ‘Let’s masturbate,’ and led him to the bedroom, pushing him gently onto the inflatable mattress.

Within a minute or two, Fiona and I were naked in our separate beds, both on top of the blankets with our vibrators inside us: me on my back, and her face-down. Ryan’s hand was inside his boxer shorts, but he hadn't taken off any more clothes.

'Why aren’t you undressed?’ I asked. ‘She wants to watch you. Forget I’m here.'

He hesitated, then removed the rest of his clothes, letting his erection pop out. He turned his head towards Fiona. She watched him back as he placed his hand on his foreskin and began his familiar up-and-down motion, and I heard her increasing the speed of her vibrator.

Like me, Fiona didn't make a great deal of noise at orgasm, but I could tell by her breathing and squeaks when her first one hit.

After catching her breath, and with her eyes now fixed on him, she switched off and removed her vibrator. She climbed off the bed and joined him on the inflatable mattress, kissing his chest and then his neck. He kept masturbating.

With one hand still on his chest, Fiona reached into the drawer where we kept our vibrators and condoms – and other sex kits.

‘What are those funnels for?’ said Ryan.

There was a short silence. ‘Maybe we’ll show you sometime,’ I offered.

Fiona took out a condom, kissed him on the lips and tore open the foil, then replaced his hand with hers on his penis, rolling it onto him.

She looked up at me and asked, ‘Are you still sure it’s all right?’

‘Do you need me to say it again? You can have sex with Ryan while I’m in the room.’

Just as Fiona liked to lie flat on her front to masturbate, she normally had sex the same way, letting the guy enter her from behind. This time, she was still on her front, but climbing on top of Ryan and lowering herself down onto him. She pulled herself close enough to kiss him and began to thrust up and down.

When my brother and I did anything sexual, we generally looked away from each other. I realised something that night. We’d originally done this so we could just feel the pleasure of each other’s bodies without acknowledging whose body. But now I wanted to watch him with Fiona, and the more I thought about looking my brother in the eye during sex, the more it turned me on.

Ryan was now responding to Fiona’s thrusts, and it took only a few minutes before his breathing changed. I saw the concentration on the face of Young Man Aged Eighteen to Twenty-Four as he ejaculated safely into Young Woman Aged Eighteen to Twenty-Four.

After kissing him deeply, Fiona lifted herself off him and returned to her bed to masturbate some more. That afternoon, I’d got Ryan off by hand to take the edge off his horniness. I was concerned he wouldn’t be able to perform for my flatmate, but looking at the condom, there seemed to be as much spunk in there as he’d left on my hand earlier.

Once he took it off and tied the end, I kept my vibrator on me, and reached down to form a ring around his penis with my thumb and forefinger. I pulled it from the base to the top, which squeezed out a little more fluid. I was licking it off my hand when I realised I’d just done it in front of Fiona.

We were all silent again, other than the two vibrators. There was no mistaking what we’d done, and no excuse in the world could explain it away.

‘How… close are you?’ asked Fiona. ‘With each other. I’m not going to judge you, I’m just… I saw that and… I just want to know.’

There was more silence. Us ladies kept our vibrators switched on. ‘I didn’t mean to do that in front of you,’ I said eventually.

‘Seriously, I won’t judge you, I don’t care what you do, but just… how much do you do together?’

‘We…,’ I started. ‘We do… well, everything, but just every so often, just when one of us needs to take the edge off it.’

‘What did you say that for?’ asked Ryan.

‘Oh, come on. There’s no point in denying it now.’

‘I see,’ said Fiona.

‘You’re not angry, are you?’ I asked.

She shook her head. ‘No, just… a bit… taken aback.’

There was more silence, but the vibrators kept a steady hum. Tasting my brother’s spunk had turned me on a little more.

‘Look,’ I said, ‘as our secret’s out anyway, maybe you could help me?’

‘What?’ said my brother.

‘Just give me a helping hand.’

‘How?’

‘You don’t need to pretend. You know exactly how.’

He looked around at Fiona, who seemed to be focussing more on her orgasm again. When he turned back to me, I removed my vibrator.

‘I’ll do it on one condition.’

‘What’s that?’

‘You stop with this Young Man business, both of you.’

‘All right,’ I said, ‘I promise.’

He turned back to Fiona, who was again working her way to another orgasm, but she was able to give a nod.

‘Good.’

He cautiously placed two fingers inside me, then slowly shifted them around to find my favourite spot. Finally, he replaced the toy on my clit.

I noticed Fiona was discreetly trying to watch what was happening. ‘Move out the way,’ I said, ‘so she can see.’ He repositioned himself but kept his hands in the same place.

I touched my own nipples and it didn’t take long to bring me to my orgasm. When I was ready, I would always slap his arm gently. This would prompt him to remove his fingers as I couldn’t ejaculate with anything inside me.

On removal, two days of built-up fluid shot out of me, covering Ryan’s naked body. He offered me his fingers to lick them, and Fiona watched us intently as she had another near-silent orgasm. Normally, I would have gone for another climax, but I was exhausted by that one.

‘That was… unexpected,’ said Fiona, ‘but, you know, actually quite… nice to watch, if that’s not too weird.’

‘Okay,’ I said, still struggling to catch my breath.

‘You… always play safe,’ she continued, ‘with each other, right? I mean, using condoms and getting tested and stuff?’

‘Yes, of course.’ I said quickly before Ryan could chime in – because that wasn’t true. My brother had been finishing inside me for some time now and I wasn’t sure when he was last tested, if ever. He was sensible enough not to disagree.

‘I think we all need to talk, though,’ she said, ‘but I’m going to try for another orgasm just now.’ That was unusual; I would normally masturbate twice as often as her.

I pointed at Ryan, ‘We also need to talk – in private.’

‘We should definitely do it again,’ he said.

‘Oh, we will,’ I agreed. ‘Don’t worry about that.’