I have had my niece since birth and have raised her as my own, now that it is time to register her for school my sister has been playing this paper trail game and not even anting to give me temporary custody of her. The bad part about it is she has been collecting state assistance and child support for her, when I have been the one taking care of her from day one. What steps would I have to take in order to get full guardianship or legal custody of her?

Family court. Hire a lawyer because you won't be doing it yourself.