Legal Question in Elder Law in California

Absolutely not. Once a person is mentally incompetent, they are incapable of granting a valid power of attorney. The only way anyone can obtain power over the person or affairs of a person who is already mentally incompetent is to be appointed the conservator of the person by the probate court. Furthermore, even if the power was given when the person was still mentally competent, it expires once the person becomes mentally incompetent because a power of attorney is only authority to do legal acts that the grantor of the power could do themselves. Once the grantor becomes mentally incompetent, they cannot do legally binding acts and therefore neither can a holder of their power of attorney. If your sister is exercising control over your mother's affairs, especially making personal decisions for her or very especially if she is doing anything with your mom's money or assets, under a fraudulent claim of power of attorney, you should contact your local Adult Protective Services and report possible elder financial abuse.