Legal Question in Family Law in Maryland

Your sister cannot transfer custody to you without the father's consent, as he has joint legal custody. If your sister feels that either she or the daughter are in physical danger based on the current situation, she can file a domestic violence petition which could result in a stay away order or some remedy that protects either or both of them. She should go to the nearest District Court of Maryland and get the necessary forms from the clerk who handles these cases. If this is just a matter of them not getting along or a dispute over how things are being handled with their daughter, there are mediation services available which could help them resolve the issues for the betterment of their daughter. If there is a court order dealing with custody and visitation that she thinks he's violating, she could take that up with the court that issued the order.