My mom had my sister as power of attorney. Mom got sick was in hospital rehabilitation. Wanted to sister would not let mom calls our adopted brother to come near at hospital. Showed up with a new poa paper giving poa. He has stole money from her now he has sold her house with my sisters name on house. A lot more to say but no time. We called all the 800#no can’t do anything for you.
https://www.agingcare.com/articles/elderly-financial-abuse-exploitation-prevention-137716.htm
You should contact a lawyer who specializes in elder law. You may have to sue for guardianship. It's not gonna happen tomorrow, sorry. Good luck!
As for Power of Attorney, your brother is over stepping his bounds, as a Power of Attorney is only used when a person [your Mom] is unable to make correct decisions for herself. If Mom wasn't unable to make decisions, then she wasn't able to sign a new Power of Attorney.
I would quickly make an appointment with the "Elder Law Attorney" to get this straighten out. Whatever equity your brother received from the house needs to go into your Mom's bank account. And if he did sell the house and had forged your sister's name, well, law enforcement will be knocking at his door.