Disabled 18 yr old with kid and controlling parent. How to go about things legally in this case (long details)?

My girlfriend suffers from SMA (Spinal Muscular Atrophy) which means she can't move her body too well. She is wheel chair bound and cannot open or close her hands but she can feed herself. We have a 5 month old daughter together. When she was born we signed for my name to be on the birth certificate but somehow (and we believe her family is behind it) its not on it. Her mom holds on to the babies birth certificate and social security cards etc. When she asked for our daughters birth certificate her mother wouldn't give it to her. I was living with them up until yesterday however she is still at her mothers house in Long Island and I'm down in the Bronx. Her mother told her her SSI was cut off but yesterday we found out her mom was really taking the money and claiming it as rent for us to live there despite the fact her mother is the one who forced her to live there and wont let her leave and being shes wheel chair bound and needs somoene to push the wheelchair she cant leave on her own and i cant bring her without a car. Her mom has been stealing her SSI money, using our child and my gf's sister child who doesnt even live there to claim taxes, put them on her section 8 get food stamps for them and all that. Originally I wanted to go down to the police and let them know that she is being held against her will, her mom has been opening her mail is taking her SSi doing tax fraud and welfare and section 8 scams. But if I go tot he cops out here theres no way I can oversee her getting out of her moms house cuz her mom is an extremely tricky person. So I was planning to have my brother drive me down ni the middle of the night and her brother would open the door for us and we would sneak her out with our daughter. But after this were not 100% sure on the neccessary steps to deal with her mom and get the birth certificate and have it changed and have the SSI transfered and all that being her mom also holds on to my gf's papers and refuses to give it to her which makes it hard for her to do anything