I had POA then I went on vacation and my brother got a new POA naming himself. Help?

I had been taking good care of mom at her home. When she entered rehab for a broken hip I went home (1500 miles) tosee my family, while gone my brother went to the rehab with a lawyer and got new poa in his name. I've been taking care of mom and dad (until he passed in2014) for 3 years I did this, my brother lives only 15 minutes from mom's house but never showed up for more than an hour per week. Now he is telling me to leave mom's house, he's taking her to his filthy home and paying himself 600$ per week to care for her, there is no pay stipulation in document. Also when she signed in she has full blown dementia and I asked her if she signed something and she said no. I got copy of it from fidelity where my mom keeps approx 200 my brother filed the poa at every bank my mom has funds in and has now moved my mother to his place, again while I was on vacation. Can he do this? Have someone sign a document while clearly written on hospital discharge papers and she has dementia and is confused. ,, help!