I agree with Attorney Brinkmeier. As your mother's attorney-in-fact, your sister has no obligation to inform anyone about your mother's care, her health, or her residence. She is completely in charge. I wish you the best in resolving what is obviously a very upsetting situation.
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I'm somewhat confused -- we have no such instrument as a "Power of Attorney for Personal Care" under Massachusetts law. We do have a Health Care Proxy, which only comes into effect in the event that a person is determined to be incapacitated by the treating physician, who must put that determination in writing. But if your mother is competent, she has the right to choose who assists her in managing her care.
The health care agent is under no duty to inform others about what she is doing. However, if you believe that your mother's needs are not being met so that her health or safety is at risk, and if you cannot get other family members to play go-between, you may wish to consider intervention through your local elder protective services agency.
I agree with the other attorneys who have answered the question. But it does appear that your concern is more with Health Care than the finances. In either case, your sister is not obligated to give you notice, but if there is any question of mishandling your mother or her finances, then there could be other recourse. A power of attorney owes a high duty to manage assets in a proper manner. The health care agent is also obliged to manage your mother's health care as she would herself, if she were able to do so.