My mother has dementia and I live 300 miles away from her. I also have some health issues that won't allow me to help with her as much as I'd like to. My brother and sister need more help and ask me to come get her, and as of now I can only make it happen 2 times a year and have her 2 weeks at a time. I do what I can to help at this distance like taking care of her finances, checking account, paying her bills and dealing with Medicare and Medicaid and Veterans Assistance she receives from my deceased father's time in the service. I also take care of her home when something needs fixed and get someone out to her home.
Please realize not everyone is cut out to be a hands-on caregiver, and there is nothing wrong with that. Here's an article from Aging Care that for me made me feel soooo much better about myself. https://www.agingcare.com/articles/not-everyone-cut-out-to-be-a-caregiver-162192.htm scroll down pass the ads as the article continues.
The distance is far enough that it would chew up a whole day just getting to her. And you DO take her for "respite" and do a lot of other things.
Perhaps a group email to the other sibs, explaining to them that you are doing the best you can would help. Often, communication is not the best among families and things are said that get passed along as "truth" when in fact, they are just mis-informed.
Don't kowtow to the others, keep doing what you can/are and feel glad that you have sibs who do care. You are doing a LOT!
Have you considered memory care or a live in caregiver. Unfortunately her dementia is only going to get worse. My mom lived with me for 2 years, I started to have health issues and wasn't healing due to all the stress in my life. Mom is now in memory care.
Does the family have a plan for the next steps. Sounds like its time.
If you're paying the bills, that really is a lot of help. Personally, I was only able to be there for my Dad and handle his finances - and I understood that my sisters were still working and unable to do much.
Your siblings cannot make you feel guilty - you're allowing them to do it. Make a list of what you do and you should start to feel better. Not everyone is capable of the time constraints of care-giving.