Grandmother and father both want full custody. Who will most likely get custody? What should we do? And where do we begin?

My sister passed away exactly one week ago. She has a ten year old daughter that she left behind, named Stacy. My sister and mother raised Stacy for all her life. But now that my sister passed away, Stacy's dad wants full custody. He has a record of violence, but he says that does not matter since it's erased from his record. He is a violent man and I bet the only reason he wants her, is so that he can stop paying child support and maybe collect money from my sisters Social Security. What should my mom do since she wants custody? Any advice will be greatly appreciated. Her dad is violent, greedy, and abusive & Stacy has told us she has seen him hurt her mom before. When my sister was alive, he would only have her on weekends. But now that shes gone he wants her for good.