A power of attorney is voluntarily granted by a person, so 1) she'd have to choose to give this to you; and 2) she'd have to be mentally competent *when she gives you the POA*--a mentally incompetent person cannot grant a power of attorney (just like she cannot enter into an enforceable contract). However, if she is already mentally incomptenent due to mental illness, you may be able to have a court declare her officially incompetent and appoint you her legal guardian or conservator (both terms are used). You would need to file a lawsuit in family court and present medical/psychiatric evidence (such as testimony and reports from doctors or pyschiatrists who have examined or treated her) of her lack of competency. If you wish to explore this option, you are strongly advised to consult with an elder law attorney. Good luck.