In high school I would hook up with my best friend's sister all the time. She was 2 years younger than us also. But then again, my friend would always hook up with his sister's friends. There were many a nights where I would be in his sister's room with her and he would be in his room with someone. probably hooked up with her for almost a year before we had sex. Just one night she out of nowhere she got on top of me and then you can fill in the rest. It ended when she got a boyfriend and they got serious. The good ole' high school days.