Neice wants to live with Grandparents instead of mother?

My neice has been practically raised by her grandparents her whole life. She is almost 13 and her mom just let her live with her grandparents for 2 years while she went to school. Now she wants her daughter to live with her and continue in a different high school. She does not want to and has no attachment to her mother. Her mother lets her do what she wants and has an extremely unsafe house. She is the queen of hoarders and her daughter does not like to live like that. She can't even cook herself a meal because of the stove being piled with stuff. She wants to live with her grandparents who always make sure she has school clothes and a good meal and does her homework. Her mom wants her with her and is jealous that her daughter prefers other people. Any advice on this? School starts in 3 weeks and she isn't even enrolled in the new school yet nor does she have any of her school things together at her moms house. Her mom is unmotivated to get her ready yet is firm about keeping her there. the father has nothing to do with the situation. The child is a mess and is very confused about everything. Any suggestions would be appreciated.