Is change of address legal with power of attorney

[symphony]

My father-in-law can no longer take care of himself so my husband and I have moved in to take care of him. My sister-in law lives in her own place, but has a credit card to handle monthly bills(phone,electric,etc.) and has most of them on auto pay. She still comes by to pick up copies once in awhile. Recently we got notice from the post office that two change of address forms had been submited. One was for my father-in law, and one was for everyone in the house. My husband called and cancelled the change of address request. I suggested he call the police, but he said she has power of attorney and could argue that she has the right. Does power of attorney give her the right to do that?