Lovely posts on here. I too wish your wee family well.
Unconciously you've been pinning your hopes on that one granddaughter being non-disabled, but I'm disabled, since birth. Its just different. That's not to minimize the consequent extra heartaches and the extra work that your son and D-i-l will have to cope with. If it's helpful, think like this: when Rose (on strictly) was born, I'm sure her gran's friends said to her gran what dreadful news. But look what she's achieved & what a happy woman she is.Your dear grandaughter who's just been diagnosed might not regress like her elder sister - that's not in the nature of the disease for every child, and your elder grandaughter may well have autism as well as MD1 - it seems more common in children with other conditions.She will have challenges, but everyone is different and she might just show the world what someone with MD1 can really achieve. It might not happen this way, but it could - don't lose your positivity at this point although I know its being sorely tried. It must make it worse not being able to see them regularly and see them enjoying the things that they enjoy, and being their own personality no matter in how small ways. There are some brilliant suggestions for practical help on here, & wish you all well in such a challenging situation.