One of my college roommates (then 19) invited his sister (then 18) to stay with us for a weekend when her school played ours. I gave up my bed for her to stay with him (we shared a room) and I crashed on the couch. Everything was normal until the last night she was there. I woke up at 2am to a loud thump and the unmistakable sound of the bed creaking. Then I heard my roommate moan his sister’s name. I was so turned on that I felt light-headed and my cock immediately got hard. I pulled down my pj pants and started stroking myself. They went at it for about 15 minutes, moaning each other’s names, talking dirty. Finally, I heard my roommate tell his sis that he was gonna cum and groan. I blew my load then, cleaned up, and passed out. She left the next morning and we never discussed it.