Mom drew up a PA many years ago when my dad was alive giving my sister and I joint power of attorney agents for her. Once my mom needed care, my sister took her to a lawyer and got another one with only her as PA, and mom was beginning to be very forgetful and some confusion. Does that new PA make the initial one void?
POA's change hands like crazy. Esp as time, relationships, emotional issues crop up.
Sorry sis is being this way. Did y'all have a falling out? In honesty, people generally DO NOT understand the role of POA.
PLus which, it is a pain and a half.
If you have issues with sis doing this (and admitedly it was kind of sneaky) TALK to her.