It sounds like your Mother received part of your father's income tax return because he probably owed her past due support. YOU have no claim for the back support your Father may owe your Mother.
The answering of this question is just friendly advise, and in no way legal advise. For legal advise you should contact an attorney with detailed information about your situation, so he or she can better assist you.
It looks like that your Mom is being paid the "arrears" that are owed to her for past due Child Support. The money she received from the IRS is money that is owed to her for and during the time that the Child Support was not paid when you were a minor.
Even though the ongoing Child Support requirement has ended, the arrears are still owed to her and can be enforced through the Courts to be paid.