How can I obtain joint custody of a parent with dementia? My brother has taken my mother to his house and will not permit me to see her.

Your brother cannot deny you the right to see your mother.  You can file a guardianship with the courts to ask to become your mother's legal guardian.  The guardianship would supersede the POA that your mother signed. However, this is not something you can do your self and you will need a lawyer because it involves filing a complaint and order to show cause with the court.  If you are in Ocean or Monmouth counties please call my office.