Suggestions needed creating my own scenario

I would say set the age as a teen. I don't really play with teens but it seems like if she were a young adult, she could (possibly) more easily get away from the situation. However if she were a teen, she wouldn't have very many choices other than "running away" as a child. Also you would have more time to develop her skills and traits and all that, leading into young adulthood / adulthood. If you are going to make the girl an occult, it would have to be something where she would not have the strength to overpower the witch, unless maybe you wanted her to study in secret and get stronger and eventually be able to just destroy her abuser. Which - that could be one way she could gain her freedom. Maybe the evil witch and her children enjoy mischief of some sort, maybe they have deviant aspirations or...I don't know, something that makes them evil lol. I'm a sucker for love, I love love, but at the same time I wouldn't just jump at making finding romantic love the thing that sets her free. It would be cool if she grew strong enough to free herself. Be a strong powerful woman who don't need no man lol. Anyway, those are just some ideas. I really like the story you are coming up with! Very cool!