Please consult an attorney in the state in which the divoce settlement was entered.  There are way too many issues here to unravel in this forum.  First, there is a statute of limtations issue. that is the time with in which you are allowed to bring the action against him.  The divorce settlement is a contract and a violation is a breach and you would sue under the theory that you are a third party beneficary of the contract.  next, if your Mother sued him for the money owed then that would be a plus because she could just collect on the judgement even from his estate.  Ask her about suing as well here if she paid them out.  you have to have had actual out of pocket loss yourself to sue.  There are low income legal services but consultations at say a bar assocaitoon legal referral service should be free or low costing.  Good luck.