You really need to talk with a lawyer on this. There are two issues here--the validity of the Judgment, and the possible applicability of insurance (I understand from your question that your niece did have insurance on her car, but that she was driving an uninsured car at the time.) You niece's insurance on the other car should apply to this situation, and that carrier should be advised immediately.
There really is not enough information here to give a complete answer. You would not be personally responsible for any accident he had while driving a vehicle that he owned. However, if the other participant in the accident is able to obtain a Judgment against him in excess of his insurance coverages, it is possible that some jointly owned marital assets could be subject to that Judgment. That would depend on the nature of the assets, ie. whether they are exempt from execution or not (such...