Can another person challenge a POA that was made when the person making it had full capacity?

No. However, if you suspect the POA of wrongdoing, you can take your evidence of same to APS and ask for an investigation into elder fraud.

The burden of proof would be on the person challenging it if they're claiming the person granting the POA wasn't competent at the time. That'd be pretty tough to do absent a doctor testifying as to when incompetence was diagnosed.

If they're accusing you of abusing your duties as POA, they'd also have to prove that. That's why you always keep meticulous records of money spent on his behalf.

Background info on Rob's situation:

https://www.agingcare.com/questions/i-have-power-of-attorney-for-my-dad-with-dementia-and-my-sibling-who-has-not-provided-any-support-468160.htm?orderby=recent&page=1�

Is this what your brother is threatening, to challenge the POA?  I recall from your first post that he was threatening litigation.

I think if there is suspicion (and subsequent proof) of abuse (financial or otherwise) then this may support a case to challenge/remove the current PoA, but only a lawyer could answer that based on the quality and quantity of proof.

Sure. A person can challenge the document if he thinks the document has been improperly or falsely created, or the principal was forced into signing it. However, if the principal was totally competent and understood what the document meant at the time of signing, seems to me that the effort would be futile.

The answer would be "no". If a person has full mental capacity at the time someone was named as their Power of Attorney, then that person had a right to name whomever they want.