Legal Question in Credit and Debt Law in Pennsylvania

There are a couple of dynamics here. First, your father was the primary borrower on the card. If the father allowed sibling 1 to use the card, as between father and the credit card company, the father is responsible.

When father died, the credit card debt became a debt of the father's estate. If father did not have enough assets in the estate, then the credit card company may get very little or nothing.

I have no idea what sibling 1 signed. If sibling 1 agreed to become financially responsible for the debt, then sibling 1 may be bound by whatever she signed. If she verbally agreed, then that is between sibling 1 and whoever owns the debt (either the original creditor or junk debt buyer) and the collection agency. It should not concern sibling 2 at all. If sibling 1 did not pay, then she can deal with the debt collector or original creditor/junk debt buyer.

You say this was fraud. If the charges were made while father was still breathing, sibling 1 was using the card with father's permission I presume. If the charges were made after sibling 1 knew of father's demise, then sibling 2 could rightly dispute with the credit card company any charges which were made after father's death. Sibling 2 could execute a fraud affidavit and it would be up to the owner of the credit card as to whether they wish to pursue criminal action against sibling 1. Sibling 2 does not have to tell sibling 1 about the fraud affidavit, so if criminal action is pursued, sibling 2 can deny knowledge of this. And since sibling 1's spouse has made a threatening comment, if he is in Pennsylvania, he could also be charged with making a terroristic threat to sibling 2. That still does not excuse the bulk of the charges made prior to father's death.

Since the estate has few assets, claims must be paid in order of priority. Once the funds are exhausted, they are exhausted. If I were sibling 2, I would write a letter and mail it certified to the collection agency or owner of the debt. I would indicate that father had the credit card account and that father is deceased (include death certificate). Indicate that sibling 2 was appointed as executor (include the order granting letters). Indicate that the estate has no assets and that the account should be written off as father's estate has no resources with which to pay. Explain that neither sibling 2 nor other heirs of father are financially responsible for the debt. If sibling 1 signed some document (include the document) then advise the collector/creditor to pursue sibling 1. If no document was signed, advise that no charges were made after father's death and offer to sign a fraud affidavit with regard to those charges. In either case, it will be up to the collector/creditor to pursue sibling 1 and will be out of sibling 2's hands.