CHILD CUSTODY

My niece is being taken care of by her dad's mom. Her grandma has custody over her due to her parents caring more about illegal things. Her grandma makes sure she's fed and goes to school. But hygiene wise, health wise, she's lacking. My niece is 7 and has 10 silver teeth already, she only eats hot dogs and mcdonalds. My brother wants our dad to have custody cuz he knows she'll be taken care of. The issue is, yes he is the dad but doesn't have custody, he is in jail so can't really take his mom to court and have the custody rights given to our dad.. What's another way to go about this? Or shall I say the best way? If there is one.