Mmm. Well, first of all, you have to remember that it's her life in the end and she's got all the right in the world to date the douchiest guy in existence if she so likes. Not that you have to approve, because I certainly wouldn't in your shoes. If the guy's actually a flat out a-hole, then it's up to you to talk some sense into your sister. Provided you do it in a civil manner, of course. She won't listen otherwise, very likely.
But other than that, if it's just a matter of the guy doing something you don't appreciate, it's just something you can bring up or subtly hint at to change.
...Then again, at the end of it all, it could be jealousy, which is another story altogether.
100% a beautiful girl. Honestly!-Kris
◊ looks over at Minny and others with her, before walking over.-
Kris: Um... Hey.
And it's so easy when you're evil... This is the life for me, the Devil tips his hat to me...

Ahhh. I can relate, I can relate. Then again, my sister hardly ever gets dates...
So what's he like? Typical bad boy? Outright abrasive asshole? Or do you just not like him for reasons even you don't understand - even if he's an alright guy?
100% a beautiful girl. Honestly!