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A person may revoke a power of attorney at any time, and the revocation must generally be in writing with a copy issued to the agent under the power of attorney as well as any facilities or banking institutions where the POA may be on file or potentially used. Sometimes a person may contest that the principal had the requisite mental capacity to revoke the power of attorney, and for that reason, the principal may obtain a medical opinion in writing that she does have that capacity at the time the revocation is executed.
Once the old power of attorney is revoked, a new power of attorney may be executed.
Regarding bank accounts, because the accounts are joint, the aunt may claim that she had an interest in the money contained in those accounts and may sue civilly for a portion of the money. Or, the bank may refuse to allow your grandmother to close the accounts without your aunt's signature as well.
I hope this answers your question. Please let me know if you have any follow-up questions.