My old friend/roommate had a really hot older sister who was always hanging out at our place. She was a really sexy, fun girl who was really into going to raves/edm concerts and would sometimes invite us with. Her and I were constantly flirting with each other (well I would always flirt with her) but she always had a boyfriend so that's where it usually ended.
But at some point she and her boyfriend came home late to hang out after a night out but it was just me at home. They joined me in the living room watching a movie and eventually started making out. At one point they apologized to me and I jokingly said something like "hey, don't mind me". My roommates sister said "oh yea? maybe he can join in?", i thought she was joking but her boyfriend took her shirt off and she just looked at me with this sexy look that immediately made clear she was not joking around.