Is this refusal to eat or drink anything a sign that the end is near? Can we do anything to help her?

Like XenaJada asked, are her extremities (tips of fingers, toes, arm, head, neck,etc, discolored)?

hug!!
be very careful about stopping medicines cold turkey. that in itself can cause a lot of mental and physical problems.

i mean, that if she would take her medicine regularly, she might also have more appetite/want to sleep less.

hug!!!

Let her rest, she is being called, transitioning to a better place.

The end is near. Nothing will change her destiny. She isn't dying prematurely. She has already surpassed the life expectancy of most women. Do not disturb her, let her go in peace. I hope no one will come out with the brilliant idea of having her checked for UTI.

how do her fingers and toes look? Are they pink or dark reddish-blue?

I'm curious what you were expecting the MC to do with a 91-year-old with advanced dementia, and why she was removed.

As a person uses less energy they require less to eat.
If she is sleeping a lot she is not burning calories like she would if she were moving around.
Offer food and liquids. But do not force.
Is her food pureed? And her liquids thickened? She may be having a problem swallowing. (if she coughs after eating or drinking that is a good indication)
Call the Hospice of your choice, interview them on the phone first, then have them evaluate mom so determine if she qualifies. The Hospice intake will contact her doctor as well.
A Nurse will come 1 time a week, more if needed.
A CNA will come at least 2 times a week to bathe, dress and order supplies.
You will get medications delivered, all the equipment that is needed to make sure keeping her at home is safe.

As you and your brother now see, there isn't much you CAN do for your mother these days, with advanced dementia and likely approaching the end of her life where she wants to sleep a lot and eat very little. Ask her doctor for an order for a hospice evaluation, and go from there. My mother's hospice company used "Senile Degeneration of the Brain" as the Medicare approved reason for their services. My mother went into her bed one Tuesday in her Memory Care ALF and passed away one week later to to the day. She had hospice on board for 2 months prior, and they were wonderful. She had been eating less and less for about 6 months prior to her death, but the very end was swift and painless, thanks to hospice.

Here is a link that provides some useful reading about end of life signs in elders with dementia:

https://www.crossroadshospice.com/hospice-resources/end-of-life-signs/dementia/

Wishing you the best of luck with all you have on your plate.

You can get hospice in for her if the MD does judge that she is nearing the end. Yes, nearing the end they sleep more and more and more and eat less and less and less; you would be amazed how little you can eat and still live for many months. Speak with Mom's doctor. At 91 I would be allowing her to do as she likes,and would get hospice in for the additional care and support.