PLEASE HELP...I LIKE MY FRIENDS SISTER?

Well if your friend is Tony Montana, dont do it (you know how it ended in the movie Scarface lol). But seriously though. Just go for it. Don't come at her just trying to bang her, because that will piss off both the brother and sister. Instead try something a little casual, like ask if she wants to hang out with you and her brother all together. Then once she warms up to you, start hanging around with her and just her, maybe ask if she wants to go to a movie together just as friends, then move along from there. As long as you are respectful and don't try the same techniques that you use when you and your buddy are getting girls together, it will all be okay.