Your daughter is behaving like a needy infant, constantly demanding gratification and Mummy's undivided attention. Only, she's not a child, she's a grown adult responsible for someone who IS a child, Sadly, she is falling short of his needs too.
Presumably GS is vaccinated against MMR etc. Otherwise, the vast majority of toddler upsets are shortlived ; a runny nose, slight temperature, " crabby or droopy", and only last a day or two. These don't require a fortnight off nursery to recuperate.
Do you have a sense she is using him, as some other attention-seeking behaviour , a new version of her own neediness?
She's also married. Why is she seeking so much emotional and practical domestic support from you , (miles away) instead of her husband, the child's father, right at hand in the same home?
I'd tell both parents, that at 70 , with an active life of your own, you're feeling a little tired/over stretched and have DECIDED it's time for a re-set. Cleaning your own home and cooking your own meals is quite enough for a retired 70 yr old . So you have decided to stop cooking and cleaning for them.
Whatever they say, you respond with the following:
" Perhaps they need to re-organise household chores between themselves. Or, they could hire paid help once a week. As DD is planning to go back to work, NOW is the time to get that organised.
Obviously, GS in child care and DD going back to work also means that you won't be coming over every week any more.
GS 's social circle will be expanding to other children, playdates, etc.
Now GS is older, you look forward to them coming to your own home some of the time. Going forward, weekend visits will probably suit everyone better. Perhaps taking turns to host lunch, or tea, once a fortnight? In fact, there are some things at YOUR home you need a bit of help with from DD and SIL, when they visit you. While you entertain GS. "