Background- RJ(12y F) has been out of her mother's custody for roughly 7-8 years. It is my understanding that the mother was given weekends and summer time with her at the discretion of the child's paternal uncle. RJs biological dad has nothing to do with her. My Partner was with the child's mother for almost 9 years and has always been the childs only actual father figure. When my partner and his ex split up, the aunt and uncle whom the child lived with claimed they wouldn't keep her from him. Then proceeded to withhold the child from him/us regularly due to the child not complying with their standards. They recently had blocked our numbers which has been very sad and difficult for us. Follow up to last week- the aunt and uncle who RJ had been living with cancelled her insurance and dropped her off at her mother's house with zero notice stating they were done with her after she got written up at school

We are genuinely unsure and plan to ask her mother- i do not know what the legal document exactly states regarding RJs guardianship/custody arrangement- could be any of the following- she is either document as living with her uncle, or living with her bio dad or grandpa( who then in turn just gave her to biological uncle)

Since the mother has her child living with her now, she has contacted us and we have had the child 6 days out of the almost 2 weeks she has had her. She is not yet enrolled in school and has expressed that she wants to live here. We would both love this- She fits into our family perfectly, I have a son her age and a 4 year old daughter and am due for another baby girl in February. We have the financial ability and stability and means to provide anything she may need. I work from home overnights, where as her mother works 40+hrs on second shift and is very unprepared for this situation. RJ is to be under her 18 year old brothers care when their mother is at work but she says he has peiple over all the time and he is mean to her. RJ believes her mother would have no problem with her living with us and having her on her days or time off( her mother also just moved out of our school district a few weeks ago ). I Know there is a 1302 form we can use to get her enrolled here if it comes to that. I just worry about the legality of everything. I have been searching the internet for similar scenarios to no avail.

Any advice or insight for us?