When is it okay to try to stop someone from marrying somebody else?

Self-explanatory.

If you want to know what inspired this question, though, here goes:

I just found out a sister of mine is now engaged. She is engaged to a guy who has hurt her emotionally more than once. This guy has also attempted to rob at least one store with friends of his while she was his girlfriend. He even went as far as bringing a knife and threatening to kill a person at a cash register if cash was not handed over. His friends were caught, along with him, and they did their time. Many people who know this guy view him as a nice guy (even after knowing all this), but he can still be quite controlling.

I was told by a friend that they got engaged. She didn’t tell me, and she might not tell me about their marriage plans (until after they’re married). As far as I know, my parents have no idea…

Would it be wrong for me to say something?

She wants this guy and has been with him for years. She was with him before that thing happened with the robbery, stayed with him while he was “locked away”, and continues to be with him now that he’s a “free man”.

Is it wrong for me to still hold that robbery against him (and other possible robberies he was involved in)? Is it wrong for me to still hold him hurting my sister emotionally against him, too?

These things that he did are part of the past, and he might not do them anymore…

but still…

I’m worried about her. She is my sister… and she is not the type who wants to ever experience a divorce.

Am I over-thinking things? Should I just leave this alone?