Tell her not to do it. A couple of people above have given examples where the marriage hasn't worked out and I can add an example I know to the list. Two families were great family friends (but they lived in different cities) - kids get married - 18 months later they're divorced and the families are estranged as well. Turns out the guy was an alcoholic/drug user and he became abusive when he was under the influence. Like everyone has said - you can't change or help someone who doesn't want it for themselves.
And the issue is more than just the alcoholism - they guy you're describing is two-faced and his values are questionable (demonstrated by his choice to drink, not seek help, get into drunk and disorderly fights and possibly getting a girl pregnant). It seems like his true character comes through when he's been drinking - does your sister really want to be with someone who's morals/values are at issue?
Also, you owe it to your parents to tell them all of this now. Imagine them finding out later that you knew about this guy's nature but you withheld the information from them - they would be disappointed and hurt by both you and your sister.