Much of this huge thread is based purely on conjecture surrounding a now absent OPs situation:
1 the DIL is secretly telling her MIL she cant have the kids overnight,
2 the DIL said 'no' without explanation, (this has been a BIG one)
3 the OP is an unpaid childminder,
4 the OP is perfectly capable of having the DCs overnight, proved somehow by having them during the day,
5 the DIL does all the childcare arrangements and the son is working too hard to care be bothered by such trivial matters.
Hak - you keep asking for posters to come and explain their reasons for saying no to a perfectly good MIL having the GC overnight without giving any explanation and with no good reason. You're not getting a response because it's a straw man situation for an argument.
Some on the thread are defending and discussing the right to say no without explanation, (and when and how best to do it) and examples have been offered such as 'no thank you, not yet', 'no thank you, not right now', they have said they would rather the person offering doesn't push after the first polite no. NO ONE is going to rock up and say ''i told my perfectly lovely, totally capable MIL just ''No!'' when she asked to have the kids overnight, even though i have no good reason and am desperate for a break, because that's my right and that's why i did it!''. Because there isn't anyone.
Re: care for when you're in hospital, in my case for all first 3 DCs it was a friend or my mother who came over and babysat for the night. For DC4 the older ones were old enough to sit themselves (21, 19 & 16 :))