Hello GrannyGranby. What a lovely gran you are, sad for your family and wanting to help. I went to school with a friend who had MD. He went on to uni, and employment, was a table tennis whiz; always smiling. We only discussed his condition once. He wanted to pack in as much as possible, be treated like anyone else apart from the odd bit of help up and down kerbs when he eventually used his wheelchair all the time. He said his parents had created a bubble of positivity around him and I think that is what your family want to do within what is possible. Therefore I’d support positivity but not too much “masking” so that your DIL can let go sometimes.
Others will have more ideas but one family I knew who had one child in a different but similar situation accepted a lot of help from fellow church goers to be able to go to different activities to best suit each child as well as companionship/support on full family outings.
As well as a cleaner, what about a gardener if possible, even if your son is earning well enough so that the can afford as many positive outings as possible and spend family time rather than grass cutting and tidying. So many trips out will be medical appointments and a huge physical and mental effort.
Going into better weather, things like a safe garden, a playhouse and suitable play equipment would be a bonus.
You are right about not all conversation being about difficulties and when you FaceTime, talk to the children about their friends, toys, outings as you have always done.
Take care of yourself too so that you can offer emotional support with energy and flop afterwards.
Others will come along with help and you can keep this thread going by commenting every couple of days to bump it. Even just to tell us you’ve been for a walk, coffee or what you are watching on tv. I will look in to find out how you are and perhaps some suggestions once I have chatted to a friend who has been on this path. ?☕️