1) Even IF, for the sake of argument, your mother was incapacitated at the time, that fact does not give your brother a power of attorney or any other authority over her. Unless you mother had herself given him a regular POA or a court appoints him legal guardian, he'd have had no power or authority.2) A doctor's note or diagnosis does not have any legal effect by itself--in and of itself, it's just evidence, at best (and very weak evidence at most, if the doctor did not examine your mother). Only a court, after a hearing/trial, can determine someone to be legally mentally incompetent. So the note does not do anything, and it does not change the validity of her new POA, etc.