This sounds sooo similar to the situation we were in with my LS. She was with a guy who was emotionally and physically abusive for 3 years (although she denied it the entire time she was with him).
She too ended up in the hospital because of threatening suicide, and went right back to him when she got out.
We went to family counselling without her...just to learn how to deal with the situation and how to best support her.
We eventually had to back off and let her make her own decisions when it came her to relationship with him, but made our feelings clear (he wasn't welcome at our homes or any family functions, we were concerned for her safety/wellbeing [he wouldn't even let her eat some days], etc.) Because the more negative things we said/did, and the more times we tried to talk her into leaving him, the more it convinced her to stay just to spite us.
It took a long time, and their relationship got worse before it ever got better, but she eventually realized that the relationship was toxic and called us at 3am one day to help her pack up her stuff and take her home.
You can't force her arm into doing anything she doesn't want to do - that will just make her resentful and spiteful of you all. You need to be supportive of her but still stand your ground when it comes to him and how he treats her (without drilling it into her head that he's abusive, she knows that, she just won't admit it and she doesn't need anyone to remind her...she's reminded every day she spends with him). I'd seriously consider family counselling, with or without her, to help you all cope and help you to support her the best you can.
Hopefully she'll come around! GL!
ETA: He proposed to my LS too, and I was put in the same situation. I told DH just to get it off my chest, but really there's nothing you can do. Again, if you tell her not to do it chances she will out of spite. Marriage doesn't have to be forever.