How can I help my brother who is being abused and brainwashed?

[Sarah]

My mother had all three of us taken away from her about 6 years ago because of drug use and since then she has been through rehab and everything else. I moved back in after I was 18 and my sister is 5 years younger than I am and she has shared parenting with my mother and her father. My brother lives with his father who fights against any visitation with my mother other than 3 hours once a week supervised. My mother made a mistake but is 100% better now. Everyone knows this. She has a full time office job, a nice place where we live and we're doing great. My brother will sometimes sneak and call her saying that he is afraid to sleep because he thinks his dad will kill him. His father has beaten him on several occasions and then lies in court. He's gotten away with it for two years now. My mom finally was able to get my brother into a therapist with visits for she and him but his father is starting to cancel them. Most of the time after he abuses my brother. He scares my brother to death saying that bad things will happen if he tells. He's finally opening up and needs us but everything is working against us. He was taken to his doctor because of chest pains and his grandmother said it was because of my mother taking his dad to court and that it stresses him out. No one believes that my mother is a good mom but we live just fine. She had a bad time once, but my brother needs her. he's always saying how he wants to live with us. What can I do?