How to change POD of dementia person?

I think that taking someone out for a meal is a kind gesture, one which in fact may have more benefit to the individual than getting doggie treats and taking it to a groomer, although that's certainly thoughtful.

I do think though that it's none of your business who the designated heirs are.   You stated you didn't want to be Personal Rep (a/k/a Executor/Executrix); why would you feel that you now have the right to become involved with someone's choice of an heir?   That seems quite judgmental and intrusive to me. 

I'm also confused by the use of "niece" and "daughter".   If this woman appointed YOUR niece to be PR, then that person would be her daughter, assuming that this woman is your sister.   Is this the situation?   Or did she appoint her niece, who would not be her daughter?  You also state that:

"I just want some for her son who is in worse financial situation and being son, I feel should get some."    

This is the only time you've mentioned a son.   

And to reiterate, what you feel should be post death distributions is not your concern, nor do you have any standing (rights) to be involved in this decision. 

Being proxy under a POA doesn't give you the authority to change heirs, nor is the principal obligated to share that information with you.    I also think that you should be finding ways to get along with existing daughter(s) and niece(s) and work for the best support and outcome for this woman who your profiles states has dementia.  That's one of the best things you can do for this woman who trusted you.

I agree with all others. Your mother put the person she wished to inherit this account on the account itself. HOPEFULLY she notified her niece as the bank is under no obligation to do so upon the death of your Mom, and if you as executor don't do so then this money will go to the State unclaimed assets. And sit there forever.
When I became POA and Trustee of Trust for my brother he had an ex partner who was already in care on one of his CDs. When the CD came due for removal I checked with my brother to ask if this was the person he wished still to get these funds on his death. He said that it was, and the new certificate was opened as he wished. When my brother died I, as his then executor, notified the Fiduciary of the man he put on this account of the account, and I provided a death certificate. The funds were transferred to the person my brother wanted to have his money in this particular account, and now, 2 years later, is keeping him in a better level of memory care than he could otherwise have afford.
Do as your Mother wished should be done.
As to what can be done under your POA it depends upon how it is written and what it allows you to do. An "executor" serves after death. A POA serves before death and is allowed to do those things as stipulated in the POA document. Always check "lawyer questions" out with lawyer, not with forums. As you have had questions before regarding all this do know, as a POA you are allowed to spend your Mother's money on advice of how best to act FOR HER in HER best interests.

Niece took sis to Bank and had her name added as POD knowing sis had ALZ.
Now she gets all. I just want some for her son who is in worse financial situation and being son, I feel should get some.

You don't get to make changes because of your personal opinion.

Your sister, when she was of sound mind, made the choice to leave this account to her daughter after her death. Leave it as she chose it to be.

Niece can not touch the account until her mom is dead, until then the account is used for your sisters care and life.

POA does NOT give you any authority to enforce your will or desires on your sister. In fact doing so, against her obvious choices when she was of sound mind, puts you in danger of being prosecuted for violating your fiduciary responsibilities.

On Aug 7th, you asked how to add someone as a POD. Now you're asking how to remove someone from POD. The answers you got then still apply. Are you not satisfied with the responses you rec'd then?

The niece may not even know about the POD. Is this her daughter? Why do you want to interfere in this arrangement? The POD doesn't give her any access to this money now.