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?