Just got home from vacation with my SO and her family. I shared a room with my SO and her 18 year old sister, I’ll call her N. I had to sleep on an air mattress because my SO’s parents wouldn’t let us share a bed… so before bed I was talking to my SO and say her sister spread open her legs and flash her red panties real quick. It was pretty apparent that she “accidentally” did that. So I go lay down and my SO falls asleep( she sleeps like a rock and never wakes up) so I’m laying there thinking about what just happened and boom, raging hard-on. So a little bit time passes and N gets up to go change to her night close, she walks right over me and my hard cock, she must’ve saw that I was hard. She came out of the bathroom and “fell” over the mattress and touched my dick, she stayed there for a minute before getting up. That was the most for the first night.