Speaking as a father of a 17 yr old there really isn't anything you can do. I don't know how old either of them are but obviously they are still young. When we are young it's rare we know what type of person we want to be with and we need to go out and experience different people. Unfortunately for you the most attractive people when we are younger are generally wild and exciting. I don't know him but the way you describe him makes me believe that.

That being said he may be the love of your sisters life however he most likely won't be. If you interfere you could cause a rift between you and her and she may not feel she can come to you if things aren't so good between her and her man. As a matter of fact if you attack his character it may force her into defending him which may lead to her glossing over things she isn't happy with.

Your best course of action is to just let your sister do what she wants and be happy for her when she is happy and be there when she is sad. If this guy is wrong for her she will figure it out eventually and she will need someone to go to for support. She is an adult (I assume) and you need to treat her like one.

BTW I really don't blame you for doing a background check on him. I make my daughter show me the FB profiles of her boyfriends. I would never forbid her from seeing any of them but it gives me insight into their personalities that might be hidden in personal meetings. I know I wasn't exactly upfront about everything every time I met a girlfriends parents when I was younger lol.