It must be very worrying and upsetting for you all Nina and I do hope your DGS is recovering well now. Not knowing any of you personally, particularly your s.i.l I can only see it from how I would react. My DD3's two week old son had whooping cough and was put on a ventilator in the Bristol Children's hospital ( wonderful place). Both parents wanted and needed to be with him and although they took it in turns they had very little sleep for the time he was there.
I understand your worry would be for your DD but there is little we as grandparents can do in these dreadful situations other than support where necessary. Had my daughter had a second, older child then I should have been willing and even grateful to focus on him giving the parents time to put their efforts into the sick child. It would have helped to get me through my own worries. My DGS is now a happy and healthy toddling 18 monther I am pleased to say.
On a slightly lighter side and I hope you don't take this amiss, my daughter has a cat and during those dreadful weeks it was the best fed, loved, cuddled cat ever. It was let out regularly and talked to like a human. The cat and I remain friends still and I don't really 'do' cats.