What should I do??

When my boyfriend comes over, my sister lays on the couch with little on, no bra, spreads her legs and twirls her hair between her fingers . What should I do? If I confront her, she is always defensive, like "NO, I DON'T!" He seems to notice that she...

wears no bra everytime he comes over. Is he attracted to her? Or is he just a guy? It really pisses me off! I wouldn't make myself mad on purpose so there is something not right about the sitution.

Instead of trying to figure out if your guy likes her or not try hanging out with him somewhere else. Your sister looks like she likes the little attention she gets and is doing it to make herself feel good or to make you mad. Remove yourself entirely from the situation and hang out with him in another room or go out more often. Try walking around the your neighborhood, hanging out in the back yard, hangout in the kitchen talking, going to your nearby park, etc. You'll be able to spend some alone time with him and get to know him better without your half naked sister around.