Ugh...unfortunately, this is probably stemming from how things are at home. She knows if she pushes enough, her parents will cave. I would probably have a pretty frank discussion with the parents about her behaviour and how it is unacceptable for her age. I would tell them you will not help her with things you know she can do, and they should support this at home as well. If you keep it up, she may stop trying to test you and stop the behaviour. However, if the parents aren't on board at home, this just may continue.