Can a relative bring a psychiatrist into my Mom's home without cause, or notice, to declare her incompetent. What are her privacy rights?

So why are you concerned about whether her privacy was violated (imho, it was). There is a petition for guardianship and if you are interested in opposing that, MIL needs a lawyer, either to argue that she's competent , or to argue that someone else be appointed guardian. If niece only has health poa, what is she doing with the check book?

Thank you for your replies. In answer to your questions about the niece. She has a POA for her grandmother's health. It was given to her because of proximity. She has a husband, three kids under 10, a job and another ailing grand parent. There is a full-time caregiver in the house, so my MIL is a lower priority for her. She shows up for emergencies. I have read her POA. It involves only end of life decisions. She did not bring this psychiatrist over for anything like that. She brought him there several days after we questioned her handling my MIL's checking account. There was no court order for an examination. The psychiatrist's visit lasted just a few minutes and he told my MIL that he felt he shouldn't be there. It makes me wonder if the niece told us one thing to let them in the house and the psychiatrist something different to get him out there. That is why I am wondering about a violation of my MIL's rights to privacy. As for the caregiver, he is paid by my MIL's health insurance.

In most jurisdictions, a judge must order that exam. So the niece must have petitioned for someone to examine MIL. It's not like the niece invited a psychiatrist friend over for tea; this must have been court ordered, yes?

Have you spoken to the niece? Who is MIL's POA? How is caregiver being paid and who supervises her? Is the niece involved day to day? Are you, or is your spouse? How did you become aware of the petition?