Well, ultimately, violence is the answer. But the police are 3000 times more equipped to apply violence than you are, if it is necessary, and they are a better judge of just how much to apply. It's their job.
Secondly, it isn't about you, it's about your sister. There's no need to personally attack the guy, since this isn't about revenge. It is about her. And she's going to REALLY be mad at you if you beat the guy up, since that makes this jerk a victim, which is more than he deserves.
Thirdly, tell the authorities (counselor, police etc), and THEN tell your sister that you did so. Tell her why you did it, and that it's because you are worried about her. If she wants to hate you, at least she'll be hating you for the right reason. And later on, in a few years, she'll realize she doesn't have a right to be mad at you for that. Heck, someday she's probably going to be really thankful you helped her; once she's older, and it's all in the past.
I know it's easier to say all this from the outside, but there you have it.