Adopting a 4 year old

Royal
Hello ! So here's my situation . My husband has a niece who he lovely dearly and treats like his own . His brother is the father but doesn't do too much for her he has 3 other kids and another one coming . Her mother is 16 with 3 kids 3 different fathers no car and her own mother is a party girl has a 2 year old . I'm not knocking them that's they lifestyle . But just Thursday my husband was to go get his niece so they could spend the day together . But he walked in on his niece sitting there watching her mother have sex . We want to legally get her out of the situation . But our work schedules are crazy right now as we are trying to prepare for our own child and trying to move out our one bedroom apartment . I would love to bring her into our home she's a beautiful little girl and full of life I love her that's my baby . Her mom isn't gonna just give her up and if we do get her we'll start from scratch cause she won't give us her clothes or anything . Has anybody went threw something like this ? I really don't know where to start I'm reaching but I'd love advice .