I think it would be much better to say: She takes what doesn't belong to her. They take what doesn't belong to them.

She takes what isn't her's? Generally the 's indicates possession. Do the answers below seem more likely if you read it like this? It's just that English rules are that you never uses the ' with these pronouns. So "her's" becomes "hers".

She takes what isn't hers. They take what isn't theirs.