Mom wants my brother to come back and live with her

My brother is 17 and he's been in my custody since he was 13 and I was 19. He has cerebral palsy which is why he's in a wheelchair. I took him in because my parents got him hurt really bad. My parents are religious zealots and thought God would cure my brother's CP. So they let the pastor take him to the front of the church, say his CP is fake and that the devil is in him and told him to stand up. Which he can't and then he had people pushing him out of his chair and my parents allowed this. He cried and I didn't know about it until he told his school who called CPS and during the investigation he was put in my custody and my parents decided to let me keep him. My mom and my dad recently divorced and my mom is lonely and wants custody of my brother back. She's been trying to get him to go to church (different church) on Sundays but he's traumatized even though my mom doesn't think he is. He refuses to speak to our parents or ever go to church again. My mom is saying she's the mom and has more rights than me even though I have legal custody. She's saying I either give him back or she's taking me to court. She had a chance to get him back during the investigation but she decided to just let me have guardianship. My brother is stressed out about everything and I'm wondering if I should invest in a lawyer