Really needing some advice

My mother & stepfather became foster parents about 10 years ago. One of the first foster children they had was an 8 yr old boy. They had him for several years & decided to adopt him. Him & my stepfather haven't always gotten along very well. Both of them have been at fault at different times. However, my stepfather is very hard on him. Last year my adopted brother & my stepfather got into an argument & my brother moved out. He was 17. After staying with some friends, he eventually moved in with his birth parents who he was taken away from & for good reason. He didn't go back to school the following school year. Several months later, he wanted to come home & go back to school. My stepfather would not let him. My mom wanted him home really bad, along with myself & my six other brothers & sisters. One of my brothers let him move in with him & got him back into school. About a month later, they had an argument & my adopted brother left & went back to his birth parents. Last weekend he asked me to come get him that he had to get out of there. Trust me, this is not the environment you would want any child in at his birth parents house. He has witnessed things you would never want anyone to see. He wants to come home & go back to school so he can graduate. My mom wants him home so bad. Once again, my stepfather says no. I'm so frustrated! He adopted this child to treat him as his own, & he's not doing it! If it were myself or any of my blood siblings, there would be no questions asked, we could come home. My mom has pleaded with him & so has my brother. He won't even let him come to the house. It just breaks my heart. He really is a good kid. And I just don't see how someone can turn their back on a child that they adopted when they're pleading for help. Any advice would be appreciated. Thank you.