Legal Question in Elder Law in California

First off, unless the POA was a "durable" power of attorney signed before your mother developed dementia, it is void. A standard POA only allows the holder of the power to act with the capacity of the grantor of the power. Once the grantor of the power becomes incapacitated, physically or mentally, such that they could not act on their own behalf, the POA expires. So unless it is a durable POA, your sister has no more rights or powers than you do. If it is a durable POA, then the powers are governed by the terms of the document. It would be a very odd POA indeed, however, if it allowed the holder of the power to keep family from visiting the grantor of the power. Furthermore, cutting off an elderly person from other family is generally regarded as elder abuse, if not by itself then as a huge red flag. You should consult with your local adult protective services department and/or elder abuse unit of the district attorney's office and see what they think about this situation.