You should talk to a experienced shrink or a psychologist about what the best outcomes for her might be. Sounds like she is dealing with what they would call traumatic attachment. Best outcomes for the individual won't be intuitive in such cases. Also there will be a psychological cost to you for opposing what your family might think is right. It's always good to be understand how to handle it before taking any stand.
This is where an experienced counsellor can make a huge difference. They usually have seen many such cases and have a sense of possible positive outcomes.