Is there a way to relieve the #2 POA of their obligations?

I think you are the one who misunderstands the role of POA and the role of the alternate POA. POA does NOT equal caretaker. POA is for making medical/ financial decisions according to the terms of the document. Being a caretaker sounds more like what you are complaining about. Caretaker can means all the hands on work or placing someone in care. The niece accepted the role to fill in if YOU are unable to fulfill your commitment as POA, not caretaker. It sounds like you are demanding something else of her and she didn't sign up for that. If mom is still competent you can ask her if she would like to replace the niece with someone else. If you need help with caretaking, as it sounds like you do, there are plenty of posts here you can review for ideas about what to do besides berating the niece. Also, consult with mom's doctor for advice.

I see on your profile that Mother has dementia. She may still be capable of changing her POA document, if it is clear that she understands what she is doing. If she is beyond understanding that at this point, it would take a court to change things.

Does the POA list both of you as co-attorneys-in-fact? Or are you listed as the primary and niece is listed as the secondary?

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.