The thing I don’t understand from OP’s posts (am a significantly older sister myself!!) is why the boys - at 6 and 12 - are considered a package deal, but the young ones at 4 and 6 are not.
I might be naive here, but I would personally find it easier to handle two kids of roughly the same age/stage who are likely to have similar interests and do similar activities than one infant schooler and one practically a teenager.
OP, why do the boys come together but the youngest two don’t?
Why is the granddaughter the one who has to visit alone?
What do you spend your time doing when the children stay over?
The next James Bond - who is your vote for?
Wes Streeting announces we have 1,000 days to prepare for war.
have them individually or the two younger ones together this is probably not going to to suit the SiL who appears to want some completely child free time.