I don't understand. What do you mean they "beat her up when she is happy"? Is that the version of the story THEY would tell?
I do not condone physical violence in any way, shape, or form, but because she is too young to move out, if they are beating her up because of her relationship with YOU, then if you really love her, stop seeing her until she is older. It is a matter of practicality.
Somehow I don't think we are being given the complete story.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?