Mother is 91 and still living in her own home, although it is obvious that she needs help. Advice?

My brother claims he has POA & MPOA. I’m fine with that as I know it is a big job. I’m sure our Dad, who died about four years ago, help set it up for her. The problem is, brother won’t keep me informed about anything relating to her health. I can’t even drive her to the Optometrist. She has twice, knowingly, signed a HIPAA release form for me to log into her chart, and both times my brother has cancelled it. I take her shopping or shop for her, take her for rides, and bring her meals when I can. She needs help cleaning, also, which I try to help her with. She has short-term memory loss which is getting worse by the day. She also claims to be sick a lot and I know she’s depressed. She shouldn't be alone. From what I understand, a MPOA only takes effect when the patient is declared incompetent or incapacitated. How can I find this out? MPOA’s do not have to be recorded in our state. My brother will not even let me read it, and my mother is too afraid of him to show me what documents she has relating to this. He has told me that it is none of my business and keep my nose out of it. He once threatened to file a restraining order against me. Friends who have cared for their own loved ones tell me they just think he’s waiting for the $$. We used to have a good relationship until he got on this power trip. I’m heartbroken over the way he has been treating me and our mother. Advice please?