Sib revoked POA knowing that our mom had a letter from neurologist that she no longer has legal capacity to make rational legal decisions.

I was mom's POA and my sister took her to a notary and had her sign a revocation of the POA and filed it at the county clerks office. My sib had the copy of the letter from the neurologist stating mom could not make these types of legal decisions, but did it anyway. I was in the process of selling her home as mom needs money to pay for her assisted living fees. Now what?