I was on a family vacation with my wife, sharing a room with her and her siblings. We let her brother and his wife have the one queen bed because they lived apart at the time. I was in a single bed next to them, my wife and her sister on cots.
Late at night, I was trying to sleep, but I heard movement in the bed next to me. There were a few giggles and whispers, and from the rhythmic slapping sounds, it became quite clear his cock was in her hand. I tried to block it out and sleep, but I didn't want them to hear me move and freak out that I wasn't asleep. I was facing them, and opening my eyes slightly, I could see an outline of their activities. I didn't want to be, but it was making me hard and horny, and since they were really into what they were doing, I managed to slip my hand under the covers and start to stroke.
I matched their rhythm, and after about 10 minutes, I heard his breathing get louder and quicker, and then she was guiding his cock as he came on his chest. I came so hard listening to her finishing him off. Now I can never quite look at them without remembering how nice it was to cum together.