Custody issue question?

I am the unofficial stepmom of a 7 year old girl. My boyfriend and I would like to get custody of his daughter. She tells us often that she doesn't want to live with her mother anymore. She is in a house with four other kids and another on the way. My boyfriends ex wife and her fiance' don't work and there are openings around town and he is capable. They are actually living off of my boyfriend's child support. His ex is not buying much needed clothing for her and when we get her on visitation she has dark circles under her eyes. Whenever we have to take her home she starts crying, we are all very close so I'm sure that you can imagine how heart wrenching it is to see her that upset. Her mother is constantly causing problems and even her own mother admitted that her and her fiance' do it to try and make my boyfriend angry. They are living in a house that her name is not on the lease because her fiance' had to have his parents cosign because they are both so far in debt. They have a Jimmy that is only seating enough for 5 people safely but yet they squish in 2 adults and children that are 8, 7, 6, 5, and 3. According to law at least 4 of those kids need to be in a carseat which we know for a fact they are not. We don't really have a lot of money at this point but we do know that my stepdaughter is miserable, even with the new baby on the way she still tells us she wants to be with us. At her home now she doesn't have her own bed or own space. She has to decide which one of the girls she is going to share a bed with, and one of those mattress set's my boyfriend bought for her. We hear stories that she isn't getting enough to eat and see the proof when she ravages her dinner or food we have at our house. Childrens services in our area is basically useless, my question is what are our options? At what point are we able to get someone to step in and say ok this isn't right and these kids need to be with more responsible parents? If it means that my boyfriend and I would need to get married to get custody of her we will. We are planning on it anyways but want to take care of his daughter's needs first. What are her rights? At what point is someone going to listen to what she has to say? We need advice, please help us help her.