Paying my child support arrears directly to my child?

I owe my ex around $3,000 in child support from when my daughter was living with me, but my ex refused to cancel her request for child support. My daughter just turned 18. My brother-in-law recently came into a large sum of money, and was able to pay off his arrears, through the state, directly to his two adult children rather than to their mother. We are all residents of California. Can anyone tell me more about this loop hole?