Take care of your sis first. Family first here man.
Now, this guy. Very simple if he still lives at home. And the fact that he fucked a 17 year old leads me in that direction. Call his house when he's not home. From a payphone, mind you. Pass yourself off as your dad, and explain to his rents that he fucked an underage chick, and that you're ready to beat the shit out of him. Let his rents do the dirty work for you. Once that phone gets hung up, he is absolutely toast.
Now, if he doesn't live at home anymore, the answer is a swift kick in the nuts. From behind, drop the piece of shit before he knows what's up. Couple more kicks/punches, make sure he knows exactly why the fuck he's laid out on the pavement, then leave.
Mind you that, otherwise I would call that a really pussy way to fight, but, you've gotta remember the piece of shit you're dealing with. It's like the mafia, all's fair when it comes to family.
And have a bat in your trunk from now on, just in case.