To think it's unfair that SIL is having a free holiday at my expense?

[sleepwhatsleep]

We've booked a holiday home in Cornwall for a week in the summer. Me and OH with DS, his parents, his brother with his wife and 2 kids. Between the 3 couples we've split the cost.
Anyway last night MIL said that OH's brother's SIL is going to join us "for a few days" as "there are enough bedrooms anyway". There is just enough bedrooms for the 3 couples and the kids. OH's brother has 2 kids - one will be less than a year old so sharing with them anyway, and the other has special needs so needs their own room.
Mil has already referred to "well if we have to then your DS can sleep in your bathroom for a few nights" which i thought was odd as there were enough rooms for him to sleep separately (he will be 1 and a bit). So i guess from what i learned yesterday that they've already decided that DS will have to give up his room to accomdate his SIL with her kids for their free holiday for a few days out of only a week?
What's made me more annoyed is that they haven't asked us beforehand.

AIBU to kick off about this?

Well you're going to look like the bad guy for insisting your baby has a whole room to theirelves!

I wouldn't go, it's a crazy set up. As my mum is fond of saying, you're on a hiding to nothing.

sleepwhatsleep I feel sorry that you are in this situation and I think that everyone going along with this change, and not being asked before it being mentioned to the sil, would have me pulling out. The 3 parties paying shouldn't have been consulted, instead, you were put on the spot. By enabling your mil to just change plans with no regard towards you guys, she will feel it is no big deal, and will feel free to pull similar stunts in future.

Does the SIL have form for gate crashing people's events/holidays?? Does she have her own holidays? Is she skint/tactless/thick skinned?

It is an odd situation- I would love to know some background!

OP/ sleepwhatsleep - has the holiday come to pass? Update as & when!

This could easily happen in my family. (Irish) Growing up no one ever left our house without a meal, children slept on the floor to give a guest a bed and it would be considered the height of rudeness to travel through the area and not call in at a moments notice. Being expected to 'shove up and make do' to accommodate a guest is what marks you out as 'family', so in my mum's head expecting you to give up a room would actually be a compliment that shows she considers you part of the family.

Just to be clear though this is exactly why I don't go on holidays with extended family

Bit startled to read 20 pages of a thread, with virtually all of mumsnet in agreement, about a situation that seems fairly normal to me but it did make me wonder if there might be a bit of a cultural difference in play?

Annwithnoe is think there's a big difference between shoving up and making room in you're own house for someone to stay over and having to give up a holiday room that you've paid for so someone else can have it for free

I agree that there should be

Whatever happened with this?