Yet another awesome point made by the brilliant, Mistral :)
Bebo, a couple of months ago......there was a guppy who created a thread about her khala who had divorced her husband after she found out that he had HIV.....which he contracted from having a very long extra-marital affair/s. The khala never saw it coming.......actually nobody in the family did.........because they had a happy/healthy marriage.
^Keeping this example in mind......it's important to talk to this guy.....get all the facts.....do a thorough invesitgation. Just because things seem "good" on the outside.........doesn't mean that all is well.
Please share these stories that we have posted with your sister as examples. 25 is not old. There are girls who get married later than 25 and have successful marriages because they didn't "settle" for what seemed satisfactory.
Also..............IF..............IF............your sister DOES decide to marry this particular guy...............since you know about his alcohol addictiona and promiscuous past...............don't hesitate to request a blood test. I think some states might require a blood test for marriage (Mistral....any idea?)......and others leave that choice to the couple. I know my cousin and her fiance got a blood test done......even though they both had a decent reputation/background. You can't toy with your health.
As **Alvena **suggested.......your sister and YOU should get to know the guy and then inform your parents. I would recommend that you NOT tell the guy's sister about the purpose of this lunch meeting. If you tell his sister that you want to get to know him better because he has a shady past.................his sister might even help him prepare the "right answers" to give you. Or he might prepare himself as well. You WANT this guy to be as natural as possible when answering your questions so that you can get a better feel about him.
Your sister can ask her set of questions......and you can have YOUR questions ready as well. I think this is important because your sister may not be ask the right questions since her mind is already made up to marry him. You on the other hand can be more objective.
Also........after the lunch.......talk to your sister........listen to what she has to say. And let your parents know about what you know about this guy and what you've learned about him during this lunch meeting.