I assume that you have asked your sister more than once to return the coins and she has not done so. I would then have an attorney write her a letter to demand turnover of the coins immediately. Hopefully you have an inventory of them, which would immensely helpful in guiding one on the exact property to be turned over. I would appear that your sister is acting as a "bailee" for this property, and she could be subject to special damages if they've been converted or damaged.
If she doesn't turn them over, go to court and ask for an order to have them turned over. As the fiduciary for your father and his estate, I think you have little choice but to do so.
The foregoing is not legal advice nor is it in any manner whatsoever meant to create or impute an attorney/client relationship.
It is important for you to remember that as Agent for your parents by virtue of the Power of Attorney which they gave you, you "stand in their shoes." Question: would your sister refuse your own parents' request or demand for the return of the coins? By refusing you, as agent, your sister is refusing your parents. She should be made to know that fact, whether through a lawyer letter, as my colleague suggested, or through a court order. I hope it doesn't come to that, for everyone's sake.
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