DH and MIL have a tricky relationship but on the surface are fairly ok with each other and we see her usually every few weeks (or at least he and the DC do). It's our youngest DC's first birthday next week and she was invited over for birthday tea. She messaged the other day to say 'Thanks for the invite - I suppose it's ok if I bring cousin Lulu (names changed to protect the innocent) who is staying that weekend?' She does this quite often - bringing/ inviting extra people along to stuff - family days out etc. Usually presented as a fait accompli or as a 'you don't mind if Jessica joins us that day?' while Jessica is sitting there smiling at us, so it's impossible to say no! It drives DH mad but we have always just gone along with it to keep the peace.
Anyway, this time DH has decided to say no, mainly because he thinks FIL (very acrimoniously divorced from MIL and has recently been diagnosed with Alzheimer's) will struggle if MIL comes with back-up from cousin Lulu. He can just about cope with her on her own but DH thinks an extra strange person on her 'side' (MIL and Lulu were not in contact prior to PIL's divorce so he doesn't actually know her) will probably send him into a flap. If it comes down to it DH would rather have FIL happy than MIL plus Lulu in attendance.
So, he sent back a reply saying 'ideally not as will already be a squash' (this is true, we are mid-kitchen renovation - it was meant to be finished by now and is very much not) 'why don't you two come over earlier in the day instead?' MIL argued one more person wouldn't make a difference, DH doubled down, MIL insisted, DH insisted not... upshot is MIL is furious (turns out she had already invited Lulu) and has now said she won't come at all, will post DC's card, and is telling Lulu the whole thing is cancelled due to the kitchen not being finished.
Despite not being generally given to taking MIL's side in things I think DH may be being a bit unreasonable as:
1- we obviously could fit one more person in despite the kitchen situation, so it's a really rubbish excuse
2- FIL is quite likely to bail at the last minute anyway - he has always been a bit flaky with social events and now quite often doesn't feel up to it at the last minute
3- having let the inviting random people to things go on for ages without telling her it annoys him, it's slightly harsh of DH to dig his heels in now when it's actually an event she's really bothered about.
Anyway, I am being outwardly supportive of DH because I think this actually all has a lot more to do with how he's feeling about his dad's diagnosis than it does to do with MIL or Lulu, but privately I think he is being a bit unreasonable!
YABU: DH is fully justified in refusing to back down and let MIL bring Lulu
YANBU: DH ought to pick a different hill to die on