What can be done if a POA exceeds their authority?

Question Details: My mom had an undiagnosed urinary tract infection for 2 months. The symptoms are weakness in the legs, dizzyness and intermitant confusion. She was finally diagnosed and treated for it. She is now back to normal with all symptoms gone. My brother was listed on her healthcare POA. He called her doctor and had him write a letter saying that my mom was incapacitated. He did not examine her, just wrote a letter. Does this make him POA or does that letter have to be filed in the court for it to go into effect? Mom has since signed another power of atty revoking the previous one and naming her 2 daughters as agents when she is not capable of making decisions. Is this new POA valid since that doctor's letter is out there saying my mom is incapacitated? What can be done to remedy the situation?

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