I think you and your husband need to step back fr a bit, and stop being the family default. And you need to let everyone know that that is what you are doing.
I would tell your parents the travelling to them so often is no longer possible, it's preventing you from meeting your commitments to work and your husband/children. Such a pity they live so far away, blah blah blah. Perhaps, Dad, seeing as you don't want to move closer to us you should consider getting in some paid help. Maybe talk to your GP about care for Mum as her dementia increases. Anyway, I'll phone next week, talk more then, bye.
And a quick call to your sisters - not with the expectation that they'll step up their involvement, but just to keep them in the loop. Let them know you're not going to be running up there nearly as much, travel is now too much especially as you can't drive any more. Work's getting so busy now you need your downtime, and the DC don't get to see their friends if they're always being dragged to grandparents. And how are you, how's work/your hobby/pets these days? Oh, how lovely.
Then your husband. He needs to call MIL and say he's going to be busy for the next few months. Work, you know how it is, so busy yadda yadda yadda. Won't be able to come over, she needs to arrange for her shopping to be delivered, or something. Maybe speak to her daughter about how to do that. Love you, bye. (Yeah, bit sexist assuming the only reason she'd accept from a son would be work commitments, but I'd put money on her not challenging him on it.)
Then he calls his sister, keeping her in the loop. Sorting mum out on the weekends no longer sustainable, can't keep leaving everything to my wife, want to spend time with my kids, she gets me to help her shop, could you maybe help her get online with Tesco/whatever. Thanks sis. And how are you?
And then - step back. Live your own lives.
Yes, eventually it will be realised by all that the three parents need some structured support. BUT - and this is the crucial bit - nobody is going to assume that this support is automatically your and your husband's responsibility. And at that point you, and your siblings, and your parents can jointly discuss what's the best plan for the future.
So step back now, do not be the default.