There is no requirement for a power of attorney to use FMLA leave to care for a parent, and no requirement that you have power over her medical records (e.g. the care could be just caring for her at home, with nothing to do with her medical treatment). Adivse your HR department to check the FMLA requirements (e.g. with or at the U.S. Department of Labor); if they still insist you provide this, they  will be violating the law by requiring more than the law requires, and you could contact the Department to file a complaint.