a couple thoughts.
1) is she eligible for hospice? Covered by medicaid and they tend to be good about getting supplies and dealing with her care onsite...if the goal is palliative and not curative it should cut down medical appointments. Once my mom started hospice, she only saw the hospice nurse on site.
2) even with hospice, a geriatric care manager may be helpful and without it, seems worth it, especially if her prognosis is a year or less.
3) Offer to go out every 4-6 weeks for a few days at a time. I was my mother's main caregiver and my sibling really wasn't involved in the slightest, but when he did come out, he expected me to drive him to visit her, etc. He didn't actively do anything, unless I arranged it. However, he did agree once to come out and stay in our home so we could go away and he was around, since my mother (who at that time was actually fine, physically, but horrible mentally) berated me non stop for abandoning her to go on vacation.
4) Until my mom's dementia really kicked in, she was pretty horrible to me at times. I was the person taking care of her but I also represented her loss of independence, etc. It was hard, but I had to kind of separate myself from these emotions and see her as a very scared and sad person. Then there was a period, before she really lost the ability to communicate and understand, that she acknowledged how much I had done and that she was sorry she put me through it. I still really haven't processed everything, but its okay to have mixed feelings, and its complicated with your sibling as well. Do what you can, acknowledge that your sibling still puts in a lot of work even with caregivers--there's just a lot that happens "on the ground" that no one else seems to be able to do. I was at my mom's almost daily for some reason or another, even though she was in full assisted living.