My mother named me #1 and my niece #2 as her POA's, but my niece continually shirks her obligations, blows me off & ignores my calls & texts.
She accepted the role 3 years ago, and perhaps she didn't realize the extent of it's responsibilities, so I have no ill will on that account... but I'm starting to resent her saying she's committed to caring for her grandmother, that she'll do something or come help, and then make excuses, or not show up at all.
I've been doing this on my own for a very long time now, and would like relieve her of any future expectations, and hopefully save our relationship in the process.
If you are primary, then all the authority and responsibility is in your hands. Only if you were incapacitated and couldn't act on your mother's behalf would your niece have the authority to do so.
So the first thing to do is to re-read the POA document and understand whether you equally have POA or whether you have the POA and she is your backup.
Next, check out what the responsibilities of POA are. Generally they are to act on your mother's behalf, and with your mother's funds, in financial and legal matters. You can pay her bills, or help her pay her bills (with her money), deal with the bank, talk to her creditors, etc. A POA isn't about caregiving. What kinds of things are you expecting your niece to show up and help with?
If you and niece have joint POA, the way to change that would be for Mother to draw up a new document. You cannot change the POA -- it is your mother's document.
If you are primary and niece is secondary, and Mother wants to change the secondary, again she is the only one who can do that. But there should be someone named to take over in case you are incapacitated.
Good luck.