Mom's living with me, my brother is POA, and is only sending her a fraction of her SS & other income. Can we audit where the rest is?

She has mid to late stage dementia, and I've been told it's probably too late to change anything. But isn't that illegal? Isn't he stealing her money? We're talking about several thousand per month that he refuses to answer any questions about! I feel certain that he's exploiting her and have tried APS for assistance but they don't seem to be of much help. Should I seek the help of an attorney? She's fully aware herself of what's going on and we've been discussing it daily for weeks. She has no trouble remembering any of it, and of course she is livid at what her own son is doing to her! In her lucid moments you almost wouldn't know she had dementia. In a case like this can she have her wish granted to make my brother's POA null and void? Isn't there any legal protection for a situation like this?