I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
Well, you contact the card company and you tell them to hold onto it because you are going after the person who stole the card. Do not tell them they are related (card holder and thief).
YOU SUE YOUR SISTER (SAD BUT TRUE) FOR THE MONEY AND ISSUE THE CONVICTION AGAINST HER TO THE CARD COMPANY. They may be able to work something out to collect from her and excuse your mom's estate. OR you pay off the debt from the estate and have the estate sue your sister.
See an estate atty (they can tell you where to start) and what the best bet would be. Try attorneypages.com or your state bar association's lawyer referral program.
Hope this helps.