Concerns for my neice's safety, wellbeing, and quality of life.

I'm at a loss and don't know what to do.
When my brother fathered a child he and his then girlfriend were very young, he didn't have much to do with his daughter, because her family didn't like him and wouldn't let him near her or his child.
Over time she contacted him and asked him to take her on weekends so he did. For a long time this was the only contact he had with his daughter then she called it quits. He then went and saw lawyers etc to try and get acess in place and she wouldn't agree in mediation.
When she was 2 1/2 I payed for her and her mother to come stay with me for awhile because I wanted to meet my neice. After this I didn't see her again until she dumped her on my brothers door step 2years ago. For almost 12 months she resided with my brother, because her mother was off partying and having fun (mind you while still collecting her single parent payment) In that time my brother started mediation and legal preceedings to try and gain custody of her, also took her to speech theropy and early intervention because she had speech and developmental delays. The first time in almost 12months she wanted to see her so she caught the bus to our town, met in the local park and when my brother turned his back she took my neice and ran. Turns out she had a friend drive down so she could get out of town ASAP. Because there were no legal Coustody arraignments in place and she is her mother nothing could be done to recover her and return to my brother.
In the last year and a half I have attempted to contact her to see how she is doing, to see if me and my kids can meet up with them when we go to her town and nothing she just ignores me, changes her number, blocks me on fb.
Just before Christmas I found out a number of things about her and what she is putting my neice through, she barely has her in her care and leaves her with friends and family so she can do drugs and work the corners, she has also made allegations that my brother touched her inappropriatly, so now my brother has an investigation on him for something she made up.
I have reported her to docs but they don't seem to care. Today I rang up and put a report in annonumasly And the lady said it doesn't sound like the mother is doing anything wrong and my brother is the one who should be in trouble. I was disgusted. How can I get someone to take me seriously and actually investergate her and my nieces welfare? I'm at a complete loss to think of my own family being raised in a house where needles are left in draws and other stuff on coffee tables. If there is an investergation on my brother how to I get my neice in my care until it's all sorted out and he is cleared?