My wife's brother invited me to his bed to join him and his wife for a threesome. My wife and I have a cuckqueaning relationship but are extremely selective and I have only been with two other women. One of them was her sister-in-law (her brother's wife).
I was only allowed in their bed under the condition that there be no humiliation of any kind of my brother-in-law. They would have sex first and he would finish, and I'd be allowed to be with her after while he watched and make sure she finishes as well.
It went exactly that way. I took his sloppy seconds, without a problem, after they went at it for about 10 minutes. I enjoyed her for about 20, not because I can last longer but because I tried to go slow and really enjoy it. She has been a friend for a long time so it was less lusty and more lovemaking. We all enjoyed it. We did two positions, doggystyle and missionary and I ended up finishing in her missionary style while my brother-in-law watched and held her hand.
They don't clean up after, and like falling asleep so I washed up and got up behind her while they held each other. That was interesting, feeling left out yet strangely included where I normally shouldn't be at all.
The best part of the after-sex was I actually ended up waking up with her on my chest. I got up first, got some bagels for them, got ready and left to go home to my wife.
I texted both of them thank you for the awesome night and they said they loved it. We hang out all the time and it doesn't really come up very often, just something fun we tried.
I see my wife's sister-in-law in a very very different way now, and can't really see her as being just another member of the family anymore. There are things you learn about a person once you sleep with them and that can't be unlearned. There's a certain awkwardness with me and her, and it's clear what it is - we want more of each other.
My brother-in-law and I have talked about doing it again, he told me their sex life has been absolutely amazing ever since and that he really enjoyed watching his wife as an observer because he'd normally never get to see those angles of her up close and personal if he is doing the deed himself. He told me he doesn't really feel comfortable opening up the circle to anyone else though, so I might be really lucky in having this arrangement be more regular.
Right now, have no idea where it will go. I'm lucky my wife is OK with this and sees every single safe encounter I have with other women as HER conquest. It's very unsure right now overall and we are taking it one step at a time.