How hard is it to sue a sibling who owes the parents money and the POA/ Guardian was given a notarized letter to collect such funds?

I'm not sure who the letter is from or who it's to so I can't really understand if that would be evidence. Did you sister sign something admitting that she owes the money? Are you the POA? This should be handled by the POA and not other family members. As far as I know, unless a sibling signs something agreeing to pay for the parents funeral or medical expenses then they can't be required to pay for those. The credit card company can be given the name, address, phone number of the sibling who used the card and they will handle that collection all by themselves. They are very good at that. If she signed for any purchases they can go after her. If I needed to collect money from a sibling I would first try to pay it myself in order to keep peace in the family or just let it go and move on. If I could not, and I was the POA, then I would gather the evidence and turn it over to a collections attorney. The attorney would send a demand letter. If that letter is ignored it would be followed by a summons to appear in court. If the evidence is good, that would result in a judgment against the one owing money or a continuance to a trial. A judgment can be used to legally garnish someones wages, attach a bank account or put a lien on property etc. Obviously moving to the collections phase is not going to be family friendly.

My question is, how do you expect to pay for a lawsuit or a collections attorney if you are already having to file bankruptcy? If your sister doesn’t have the money, a lawsuit isn’t going to magically make the money appear. And if you were your dads POA, it ended when he died.

the credit card company is NOT going to go after your sister if her name was not on the credit card account. They have no legal standing to do so. Your dad gave her permission to use the card(s).

Your dad could take sister to small claims court. He has a 50/50% chance only because he told her no and kept telling her to stop. But even than it would be a tough case!

Good Luck!