Can my abusive ex-stepfather really get custody of my little brother?

since my mom left him he has been trying to destroy her life and control her through their 7 year old son, who is my brother. he is a physically and emotionally abusive man, and he is A MASTER MANIPULATOR!!! he has managed to get nearly everyone on his side and convince everyone that my mom is a liar and that he is a great guy. they have 50% 50% custody, and now he is trying to get full custody with supervised visitation only for my mom!!!! my mom is a great mother, HE is destroying my little brother!!! can he really get away with getting full custody of my brother????