I'm sorry to hear all this @TizerorFizz Families are so complicated aren't they?! Your siblings are either ignorant inconsiderate asses, or have just detached themselves from care duties/closeness to your/their mother, or maybe they are genuinely very busy, CBA to travel. Could be any reason. I guess you will know better than me!
A possible reason for your sibling(s) not coming because they don't like your mother's political views (as you mentioned,) is frankly utterly pathetic, and breathtakingly narrow-minded. Imagine shunning your own mother because you don't have the same political views. Anyone who thinks THAT is OK needs to give their head a wobble!
They're not coming though and there is little you can do, other than accept it - same with the grandchildren who aren't coming. Just gently tell your mum they can't make it. Many extended families are not close, and the big TV families that all look out for each other and all pop round once or twice weekly to see 'mum' (and dad!) and have 'tea' and spend big old Christmases together are rare.
I feel your pain and woe at being primary carer for your mum, as this was me. I have one brother - a decade younger - who was about as much use as a chocolate fireguard. Did nothing for our parents, no fetching and carrying, no lifts, no shopping for them, no housework, no DIY, nothing. And yet he was the little blue eyed boy whose shit didn't stink. Came once a month for an hour, sat and ate their food, read their paper, borrowed 30 or 40 pounds (that he never gave back!) and then fucked off. He was still a Prince Amongst Men though!
I organised my parents funerals too. Sorted everything after their deaths. He did nothing, except turn up to the funeral(s.) He moved away from our county some 10 years ago, and whilst I don't hate him, we are not close. He moved to his wife's home town 45-50 miles away and now is part of her family. Only seen him and SIL, and my 2 nephews about 10 times in 5 years (not at all through 2020 and 2021 due to Covid.) Last time was August last year for SIL's 40th birthday party.