I suspect my brother is abusing his POA for our elderly mother who I think could pay for quality care. Can I demand an accounting?

You'll need an attorney or social worker investigation.
You may not like the results if you are wrong.
You may wish to discuss with your brother first what your mother's status is.
Finances are a complex issue and your mother has entrusted them to your brother. Do you really want to go over his head? Do you really understand what is going on?
You may wish to educate yourself first as to the responsibility and it's demands before you accuse.