My Brothers Sister-in-law Wants To Sue Him For Conservatorship Over His Disabled Wife. Is This Possible?

A few months ago my brothers wife had a stroke that left her disabled, and he's been caring for her ever since.
A nurse informed his sister-in-law that she could get up to $4,000 per month if she applies to be a caretaker, and she really wants this money. So, now she is threatening to sue him for conservatorship. My brother wants to ban his sister-in-law from coming to their home, but she is saying that she will use that action as a means "to prove neglect."
His wife does not want to see her sister, agrees with the ban from their home, and has even stated that they have always had a very rocky and estranged relationship for years.
Is it possible that she would be able to sue my brother and petition for conservatorship? He is in the USA, California.
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