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If someone filed a report because they thought your mother could not take care of her financial affairs because of diminished mental capacity, the Court may have filed a Petition for Guardianship or a Petition for Conservatorship. However, your mother had to appear in Court and would have had to undergo an independent psychological or psychiatric exam and medical exam by doctors appointed by the Court.
But even so, the Guardian appointed by the Court would have no right to touch money in accounts that had other names on it besides that of your mother.
You must file a "Petition to Set Aside Guardianship" with the Court that appointed the Guardian and in the Petition, you would set forth your reasons. Your sister should join in this Petition also since her name is XXXXX XXXXX account,