To make a valid power of attorney document, your mother would need the mental ability to fully understand what the document is and what it does -- and to consent to giving you power of attorney. If she's already mentally incapacitated, it's too late for her to agree to allow you to handle her affairs.

But there is another way to get this authority. You can go to court and ask a judge to appoint you as your mother's conservator (called a guardian in some states). Start with the clerk's office at the county probate court; they can most likely give you information on how to initiate conservatorship proceedings. For more information, see Conservatorships and Adult Guardianships.