You need to file ex parte for relief with the probate court. Contact a local probate attorney immediately. You don't have time to try this on your own.
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I agree with Mr. Shultz. This is a matter which needs to be addressed immediately. You need an attorney to take the appropriate action before it is too late.
I am very sorry for your loss and the ongoing difficulties occasioned by your sister's actions.
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Go see a probate lawyer immediately, Do not waste time on this online forum. You need more than we can provide you.
By the way - a Power of Attorney terminates upon death. Your sister's POA, even if she had one, is worthless now.