Unfortunately, your parents are not divorcing your STBX wife, so they are stuck as co-signers on that loan. If she defaults, the lender will go to your parents to make the payments. If I were you, try to include it in the marital community debt. That way, when equalizing everything, she will have to pay at least half of it. And you'd be stuck paying the other half. But at least your parents won't be stuck with the entire amount.