I don't really like being the guy who pretty much says "I concur with what the other two said", but they're right; you can't take responsibility of somebody else's happiness; people generally don't like being told not to do something they love, and yes that woman can do a whole lot better, but in the end of the day it's not really your choice to make, and if her close family isn't able to change her mind; then you can't really expect that she'll change your mind because of something she said; so all you can really do is stay beside her, and support her when she needs it, and make it clear that you don't approve of that man or how she's sharing all of that Personal information with him. so try to remain calm; maybe find a new way to destress, and remember; love is just a bunch of chemicals floating around in your brain that eventually stop getting produced once they get bored of the person, and maybe one day she'll stop looking at the situation like it's some lame forbidden love affair narrative and stops hanging out with the douche.
Edit: Don't be a shoulder for her to cry on all the time; that's a bad habit and let's face it the only person I know of who has that kind of patience was killed two thousand years ago.
So yeah... Good luck with that; I mean it.