I am not sure what Maryland law provides on this. You should check with an attorney. If you have concerns about your sister's honesty, that is certainly a red flag. Of course, if she uses a POA to transfer assets or to personally benefit herself, that would be a clear breach of fiduciary duties, and a court would very likely order her to return all assets to the estate.
Hopefully, all of you can work together to assist your mother (and each other) as you deal with these very challenging times.
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Wow--sounds like an uncomfortable situation. Do you by chance have copies of the documents? Regardless, I'd suggest you discuss, confidentially, with an attorney and that you are clear about the facts to support your concerns. As your mother's primary care giver, it seems that you'd want to have open communication with your sister while she "holds the purse strings." Certainly, your sister has a fiduciary duty to exercise special care with regard to funds she manages for your mother.
As the other attorneys have stated, your sister with the poa cannot use your mother's money for her own purposes without being in breach of a fiduciary duty owed to your mother. In order to find out whether your sister has misused property or funds, your mother may be entitled to an accounting. If your sister is unwilling to produce an accounting, you may have rights through the court system to demand one or at least seek a guardianship where an accounting would be required of your sister. I believe you should seek the advice of an elder law or estate planning attorney.
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