[Adam the Fish]

Hey - I hope posting this has maybe allowed you to get some of your feelings clear in your head.

I'm assuming that she's around your age, so unfortunately the advice to encourage her to move away from him immediately probably won't work - although at the soonest possible opportunity, she should make arrangements to do so. In the meantime, however, there may be some ways you can make life more bearable for her despite this.
The thing is, you perhaps have little jurisdiction (and hard as it is, definitely keep yourself restrained from lashing out physically - it will only get you into trouble, which would be awful for her) and getting someone with more authority involved might be helpful. I don't know what specific resources are available in your area, but if she'd be happy to get a teacher or other trusted authority figure involved to at least provide some resources and very practical action, that'd be good.

As for you, it's great that you care so much - and that'll mean a lot to her, I'm sure. If you can be there to support her, that's good; of course, I'm sure you admire many things about her, so make sure to tell her so regularly!

I'm really sorry you're in this situation, but I hope things will improve in the next couple of years as you and she can gain more independence from her father. Best wishes.