How can I get custody of my niece from her father if my sister passed away a few years ago unmarried?

My sister died during heart surgery leaving an 8 year old and a 19 year old daughter. The eldest was already married and on her own but promised to keep a close eye on the youngest as they were very close. The youngest went to go live with her father who I believe had something less than joint custody when my sister passed. The youngest is now 14 and wanting to come live with my family which is what my sister asked of me the day before the surgery that took her life. My sister had constant legal battles with the father because of drug and alcohol abuse and because he stalked and threatened her for years. He was only able to get weekend visitation and was supervised in the beginning. He cleaned up and when my sister passed I asked her if she wanted to go with her father and she did and because my sister never signed the will. I didn’t try to stop it. However, I have just returned from a month-long visit there for the birth of the oldest second child and found that the relationship between sisters has cooled and the father is back to his old ways. Which he even joked about when he brought her up to the hospital to meet her new nephew. She stayed the weekend with us and said her father left her alone all the time and when he was home he would just sit on the couch drinking till he passes out. She said he even drives around drunk with her in the car which is what my sister was always so afraid of. She said she is all alone and scared and wants to come and stay with me now. My husband is just retiring from the Air Force and our income is being cut in half so I don’t have the means to take him to court. He bankrupted my sister doing that. I was hoping because of her age, she would be able to testify herself and ask for a custody change. Is this possible?