Hello.
First of all, I'm sure your children -and grandchildren when they become aware- are glad for your commitment. I wish you happy and rewarding times with them all.
I'm a father of two autistic young adults (now) and they have 5 younger cousins on my side, in France; 2 older cousins in the UK. They have built up a really good relationship with their younger cousins, but not so with their older English ones, so I'll try to speak from both experiences and find something about the job of being a grandmother.
How did it work out, and what worked out.
We spent long times - 3, occasionally 4 weeks in the summer, almost every summer - in France with my mum and dad. Some of that time my sister was there with her 3 DSs (mumsnetters will appreciate), every summer. But we and them also had time without my parents without cousins - typically my sister would arrive early in July, us with my daughters as UK school hols began a few weeks later, we'd have a week to maybe 10 days altogether, then my sister would be off to her in-laws and we would remain in a smaller unit. Then we coordinated our Christmases as well (we're not nearby at all!) so every other year, everyone would be at our mum and dad's, and then the next noone and they'd have a quiet Xmas as we were all with our respective in laws.
We put in common our energies and parenting talents. My brother in law selected great board games. My sister took them sailing and treetop climbing, I taught them to program computers, I adapted their furniture as they grew, dug sandheaps for them to roll balls from on the beach, made temporary seaside aquariums (please don't jusdge our holiday tastes).
But we were long there and only a week and a half together, so off before any family tiffs had time to really flare up. As they grew there's also been cooking, surfing, paid farm work in their teens. The difference with disability grew, but so did the boys' understanding and respect for my DDs.
What does it mean for you... Don't ask your grandchildren to be in each other's pocket. I had a cousin my age that was a bestie, they are close in age but won't be besties. Have privileged times with their two families, and also special times (some feasts, some birthdays, a week away or so) altogether. build in time to just be, forget the educational programme of cultural tourism that is just relentless, why do impose our kids visit all of pompeii at age 3 (!!???)
Make it a family ritual that everyone will meet. A bit. More than once a year for more than a day. But if it's hard, not more than that. Support every adult in bringing in their talent to parenting - for example, my sister helped me limit screens for my DDs, it was very helpful, while I showed her the value of what one of her DSs did in his screen time, and that was life changing for him.
Your two are young. It's natural that a 6 year old wants you to herself. Give her that some of the time and tell her that you are all hers, some of the time, but also that our hearts are always big enough to love several people. She won't mind. She knows she's not with you all the time! Look for ways that they can be together, certain games, certain stories, and as they grow they'll learn to cohabit.
Two inspirations for me about disability when I think of my DDs. Sorry if they aren't your taste, I hope you get something from them regardless.
The psalm says "the stone rejected by the builders became the cornerstone". Maybe you have to build dry stone walls to know what that means. They are assembled like a puzzle, looking to fit the stones together and which way. When a stone is really misshapen, it's piled out of the way. The misshapen pile is where you go when you're need something different, an arch, a stile, the end of the wall, it fits where nothing fits.
John's gospel talks of a blind man, Jesus' disciples ask: "who sinned, this man or his parents, that he would be born blind?” Jesus says, “Neither. It was so that the works of God might be displayed in him". Your granddaughter can grow into someone special, in whom the works of God are displayed in some form.