My ex owes me and his older sons mom back child support. How will they do his refund if he woes two people?

One year they gave her all his refund for her back child support. And he owed me money also. She is the mother of his older son. I have 3 kids by him. When he does his taxes this year, how do they go about doing the back child support for two people? Or do they give it all to the mother with the older son?