Am I wrong?!? Niece needs to be taken away from sil?

Apart from being 30 years old and working part time at a grocery store and needing monthly financial help from her parents, she has a seven year old daughter who she couldn't care less about. She leaves her daughter at home by herself when she goes to the gym, or to go buy cigarettes. She doesn't make her daughter breakfast before school, she just lets her go hungry until lunch. I've actually never seen sil feed her. She doesn't take her to school or picks her up, let's her mother do it. She also makes her mother go to parent/teacher conferences instead of going herself. Her daughter missed 26 days of school this past year. 

She always brings her boyfriend of the week around her daughter and has even had strangers she met on the internet come to her house for the "date" while her daughter was there! Her on-again off-again boyfriend's brother is a registered sex offender and my sil lets him watch my niece. That kills me. Her boyfriend's 12 year old son was also caught molesting my niece and nothing was done about it. It makes me so sick and angry. 

 

This is TMI: poor niece got worms in her intestines and had them for weeks, complaining to her mom and grandma about them. Of course no one did anything about it until they crawled from her anus into her vagina. It was torturing my niece so my mother in law finally took a look and saw what was going on and took her immediately to the ER. She was given medicine for my sister in law to give to her daughter, but of course wouldn't do it. She made my mil do it. She was laughing to me about how she couldn't even touch her daughter after that. Anyway, mil missed a treatment and the worms came back. 

My sister in law will talk all day about how she's a great mom and does anything for her child. 😡

My husband and I are at a loss. His parents won't do anything to help their grandchild and say they would disown us if we tried to get our niece away and my sil isn't sure who her daughters father is, so there's no family on that side to help. 

My husband and I are very fortunate that he has a great job, own a home and are very stable in our relationship. We are coming to the realization that we need to step in and take our niece out of that situation. We could offer her a loving, caring home and be the mother and father figure she's never had. We also have two chikdren and they all get along great and love each other so much. My niece deserves so much more. 

Should we get a lawyer first? Beg sil to just sign over rights to us? Call dfcs/child protective services? 

Thoughts, solutions, opinions and similar stories would be great!