How do I stop my sister from letting my dad pay her mortgage?

Missed the important point of who's mortgage was in question. Tough love 2. Your father could put a clause in his will/trust stating the amount of owed money and stipulating that amount be subtracted from any inheritance for your sister. This of course assumes your father has a will and there is something to inherit. Make sure it done through a competent attorney to avoid problems later.

Bobby, the Dad took the mortgage out on his OWN home. Not paying will do nothing to the sister.

Tough love. Stop payments and let the bank foreclose on your sister's home. She may change her tune if lose of shelter is threatened. Your father might even recover some of his money.

Ok - thanks so much for your answer and you help - I appreciate it !!

YOU can't do anything about it. Since Dad chose to take it out on his own home, not hers, he is stuck with the balance due on mortgage - all $50K. Unless you have something in writing that she agreed to pay it back, your sister may be acting like a lowlife. But it's legally his debt to pay. You could MAYBE report it to adult protective services, but it's hard to argue that something that happened 16 years ago was undue influence. If he took the debt out 16 years ago, at least it should be outside the 5 year lookback for Medicaid if he needs a nursing home. It's terrible that she took advantage of your Dad, but unless she signed a promissory note your sister is off the hook. Depending on the laws in your state, he might be able to declare bankruptcy or quit paying the mortgage and play the waiting game about foreclosure, but options to get your sister to pay are pretty much non-existent. Lots of people who owe family money suddenly find a reason to get mad and quit paying. Sad.