Your facts are not entirely clear -- particularly as to how long things have been going this way, the ages of all involved, and whether there has been any involvement by authorities, including CPS.
To answer the question you actually asked, you could seek an actual custody order (maybe; you do not say what happened to the father, and what if any legal and physical custody orders are in place for anyone involved). Options include seeking guardianship, or a visitation or custody order, or even a formal adoption.
You should probably review the information and materials posted here, first:
http://www.willicklawgroup.com/child-custody-and-visitation/
http://www.willicklawgroup.com/guardianship/
Then consider a consultation with a family law specialist who can analyze the specific facts of your case and suggest what might be most effective to do what you think is appropriate for the young lady in question.
Answered on Apr 26th, 2016 at 7:05 PM