Emotional abuse to children

Brittany • Mommy to a wonderful 12yr old boy, 9yr old boy 7yr old girl 6yr old girl and 3yr old girl. Due March 11th 2023 with a boy.
My brother has a 9yr old and since he's been married (January 16') he has been emotional abusing his daughter. His daughters mother started going after court ordered child support last November. He has always given her money or bought my niece what she needed b4 that. Anyways I heard the words out of his mouth today to his daughter "if you don't want to come to my house and spend time with me I'll just go to the court and sign off on you" if i wasn't 29wks pregnant I probably would have slammed his ass! She doesn't like going over there cuz he constantly treats her bad and like property cuz he pays child support and he throws that in her face too. I don't know what to do. I've talked to her mother about it and she doesn't know what to do. Maybe the best thing for her is for her dad to sign off on her. My parents or I would still get her every weekend and would step up to help her mom financially with her. My hearts breaking for her. She's needs protected from the abuse.