Should parents talk about child support...

Should parents talk about child support infront of their children? I personally say no. My niece is 9, my brother has only been paying court ordered CS on her since April. Before that he would just give her mom money for whatever my niece needed  plus an extra $50-$100 a week. But now that it's court ordered he acts like my niece is property and when she doesn't want to go to his house he gets mad and tells her " I pay for you" im about to snap! He talks bad about her mom to her because of the child support. Any advice on what I can do to make him stop? My niece doesn't deserve to feel bad about something completely out of her control. It breaks my heart when she comes to me crying.