My niece came from Maine, rented two uhauls and took a few things from my moms house and then put my mom in a uhaul and drove to Maine. My mom was 89yrs at the time. My niece didn't tell anyone that she was doing that and my mom wasn't allowed to say goodbye or tell her kids. My brother was the POA at the time.. My niece then changed my moms POA to herself.
Only your mother can change the POA.
And if your mother is willing to do so she can appoint anyone she wishes unless she has severe dementia.
It sounds to me like neither your brother nor yourself were in very close contact here with your mother to have her disappear without a peep in a U-Haul?
Was your brother listed on mother's accounts as her POA and Signee?
Was he acting as your mother's POA.
Was other alone and safe to be so at that age?
Quite honestly, like a few questions in our threads today I have lots more questions than answers, but ultimately, anyone who remains legally competent to do so can change his/her POA at a moment's notice. In fact, withOUT a moment's notice.
If your brother's PoA authority was active (meaning it was triggered according to the criteria noted in his PoA document) then your Mom can no longer make changes to her legal documents. The niece cannot make this change for your Mom, ever. No laws allow it and no legit attorney would participate in making it happen.
Your brother will now need to go see his Mother in person and challenge this. He will probably also at some point need an attorney if there's any evidence of financial abuse.
As Alva mentioned - this is not what happened. It is not how POA works. Your mother - the principle - would legally assign a Power of Attorney. From what you have said - she assigned her brother.
At some point, either that POA was either invoked or it was not invoked. If it was invoked (by in most cases at least 2 doctors agreeing and signing off that she was incapable of making her own decisions) then your uncle is her POA, there is no changing it unless he resigns - and at that point one of two things happens. If there is a secondary POA, that person would become POA. If there is no secondary, then the courts would have to name a successor.
If the POA was not invoked, then your mother would still be considered competent. In which case SHE can legally revoke original POA and name a new one of her choice.
Let's be clear - NAMING/ASSIGNING a POA is not necessarily the same as HAVING Power of Attorney over someone (durable vs springing) . My DH was the named primary POA for his dad for years after his mom passed, but he didn't BECOME the POA(have any power) until a few months before his dad passed away because it was INVOKED.
But regardless of what kind of POA a person has, no one but the principle or the courts can change the POA. The only possible exception is if there is a line of succession with a primary and secondary and the primary resigns, then the second can become POA. But there is already a legal designation, the secondary is not just making themselves POA out of thin air.