Reply #129: It may be a better alternative than psychosis.

[Cetacea]

She is at the prime age for developing severe mental illness, especially given the environmental factors. If she is with her boyfriends family than she is probably better off. Despite your good intentions, I doubt that she'd risk getting drawn back in with her abusive family by staying with you.
I know it's a tough call, but as other posters have pointed out you have little or no legal rights here. This could actually be the best thing that she could possibly do for herself at this particular time.