How do I get custody?

I have had an 11 year old girl living with us for approximately two years now. I have guardian papers but not guardianship which I understand is different. She is a girl scout in my troop and her mother just didn't care if she never came home. On major holidays she will come and pick her up or I will drop her off for a few hours. Her contact with her daughter is sporadic at best. When I asked "Sam" why she didn't want to go home she said that there is constant yelling and fighting and her mother will say things like "She doesn't want to be a mother anymore and should just kill herself." There is another family living in their small 3 bedroom house again. Every couple of months there is somebody new living there. Of course the mother is still collecting food stamps and claiming "Sam" on her taxes. "Sam" has a 7 year old sister who does live with her mother and it really hurts "Sam" that she wants her sister but not her. The 7 year old was Baker Acted in May and unable to return to school until she received counseling which to my knowledge she still has not received. Everyone says since she is not related my chances are limited to getting custody of "Sam". I am afraid if I do something incorrectly she will come pick her up and "Sam" will be in a terrible home environment where she is not wanted or loved. Since there has been very little contact can I file for abandonment? She will text "Sam" good night if "Sam" texts her good night first but this is very limited. Sometimes it has been weeks or months without contact. I have taken and paid for all doctor and dental appointments, food, clothes, shelter and any extracurricular activities like field trips, band, etc. Please advise my next best course of action.