What will you do

The worst thing you can do for your sister is tell her that she's fallen for the wrong guy or even try to prove it to her. That's going to make her want to be with him even more, because she feels like people are against what she thinks is her happiness. In situations like these, there really isn't anything you can do, except try and be supportive of her and the guy she's seeing, no matter how much you don't like him. Maybe he's the right guy for her but you can't see that because of your own prejudcies against this guy. Maybe your sister sees something in him that you don't. If he really is the wrong guy for her, and they do end up marrying, then eventually things will fall apart and she'll get hurt. But sometimes, the only way to accept the truth is to get burned by it.