It amazes me sometimes the odd couples you see together. The best thing to do is to teach how to look for good qualities in people and trust that she will make good decisions. If she knows how to read through the BS of most guys and knows what a good guy is like, then she will be fine.
I have a little sis and she has three older brothers. We are her best friends and there to support her and give her advice. Teach her to find good guys and what a good guy is. You want her to be happy in life and she wants to find someone. She has to make her own decisions in life and life with her consequences, not you.
Being a jerk to her boy, is just a good way to push your sister away from you if she really likes him.