How do you know if a person has been legally declared incompetent?

Does your mother have a health care proxy? The doctor at the NH has to invoke the health care proxy stating she is incompetent. Also, send copy to her lawyer with regards to the Power of Attorney.

Are you asking whether your mother could change the POA or whether it would be invalid because she is legally not competent?

Can your mother understand the concept of POA? Would she want to remove your sister from the document?

Her Doctor has that information. You may or not be privy to it, if you are not part of her health care info group [ named people who are able to get her health information shared with them]. If you are not one of those people, you might still be able to get that info fro her Doc, by going through legal channels, and learning about that incompetancy diagnosis will be all you get, unless the lawyer can wrangle more info for you--say, if you are the remaining last relative who needs to take care of her or her business.

Thanx for the answers. I should have been more specific. I share POA with my sister. Her personality has completely change due to her life circumstances. Bottom line is I cannot pay and bills for Mom without her agreeing to it. My sister thinks POA means the Power Of ACTING like an AS-H----!!!! I'm sure you all get the point. I need to get Mom a new POA because my Mom's care is going to be a battle with my sister as long as "I try to do the right thing". Mom at this point is quite healthy, physically, so far. She is doing extraordinarily well in the NH lost but happy. My sister does not see the big picture at all. Mom is the easy one... her mental abuse on me is accepted, Mom can't help it!!! I hate having to pull teeth when I'm the only one doing anything and I am the criticized and honest. Bit of a vent but, I need to have total control or else Mom will suffer because my sisters a control freak that is not in control at at.

IF there are multiple persons on a POA, and they cannot agree or work constructively together to take proper and timely care of their elder, then, it is very possible to get the POA redone and assign a public POA ["disinterested 3rd party"] to do it.
As with children and pets in custody battles, the whole point of [the POA] is to take "prudent, proper, timely care, doing what is in that person's best interests" of the person who needs taken care of.
So, it often happens that those originally assigned to do that, cannot manage to do it, so a public POA can take over.
OR...the two of you might try working with a mediator....problem with that is, it is not "timely" care then.
To get the POA transferred to a public POA person, talk to a Social Worker about the details of the case.
The Social Worker should be able to assist this, or point you in a better direction.
You should not have to go broke taking care of your parent!
I do not think any State requires that--they do not want to have to pay your expenses if you go broke and then have to get welfare!
Good luck!