Without any discussion, my oldest sister has stepped in to take care of my mom, who has developed dementia. She says that for her to have the financial POA, the rest of us siblings need to submit in writing that we forfeit/waive having any control of Mom's money. I've never heard of that before. Why would that be needed if Mom has already signed control to her? Any insights?
My brother had financial POA. None of us were asked to sign anything. POA meant that he could step in and act as mom's financial agent to do HER bidding. It did NOT allow him to divest US of any interest in mom's estate.
Without knowing you and your siblings' history, I can speculative that it's past time to remedy the "without any discussion" part, with which you opened your question. Schedule a discussion with all siblings ASAP and with the help of a third-party mediator, if necessary, to map out how your mom's care (health and financial) will be provided and, most importantly, how future decisions will be made.
If your sister intends to be your mom's primary caregiver as well as handle all of her financial affairs, she will be taking on a lot of responsibility and stress, perhaps much more than she fully realizes. The last thing she'll need are siblings challenging everything she does, especially if they haven't been much involved in providing her care. As BarbBrooklyn suggested, having POA will not allow your sister to spend your mom's money on things that don't benefit her, or to change her will, etc. and POA authority terminates at death, so there's no effect on her estate plan.
Early on my dad's dementia journey, one of my 7 siblings took it upon herself (w.o. discussing it with me and 3 other siblings) to become my dad's immediate trustee, while being just a part-time caregiver. After a few years, the trustee asked me, as one of his part-time caregivers, to become the full-time caregiver, which I did and only then became aware of all the things that had been done "without discussion" that eventually forced me into becoming our dad's legal guardian and conservator. I think a lot of conflict could have been avoided (and money saved) if the trustee sister had had discussions with all siblings prior to her making trust and POA decisions for my dad. (Yes, he had dementia, but the trust attorney deemed him to be competent enough to sign documents.)
I suggested a family meeting to my 7 siblings, but the four that I most needed to participate, incl. the trustee, did not even reply to my suggestion. And I'm pretty sure that 3 of those 4 did not take much time to educate themselves about dementia, care giving, legalities, facility types, etc. I hope your decision to have a family meeting has a better result than mine did. Like I said, earlier is better -- perhaps in my family's case the suggestion came too late.