(Namechanged as SIL may be on MN!)
Some back story:
I have 3 DSs aged 10, 6 and 5.
I have a brother with twin boys aged 8 and a DD6.
My DM has told me she prefers my brothers children to mine (she always wanted twins) but recently has admitted that actually mine are not as badly behaved as she has made out, and they are perfectly nice children. Don't get me wrong - my kids aren't angels, they have high functioning autism syndrome but they try their best.
However, because of this preference she often cancels on me e.g. we were going to stay with her one summer and then she said not to come as she had invited DB and his kids instead. Or she suddenly decides not to come to stay and goes to stay with DB instead (we both live 7hours drive away from DM, but apparently it takes longer to get to us).
Every year since brothers twins born they have gone to DMs one year, and she has gone to them the next. I have offered to come to us, but it has never materialised.
The AIBU
This summer, when I was staying, she said that my brother wasn't coming this year. So I said maybe me and the boys could come and stay as they had never seen her on Christmas day before. She agreed. A couple of weeks ago, having rechecked it was ok, I told the boys they would be going to granny's for Christmas - having autism they need preparing in advance.
DB and DSIL are now not very happy as DM apparently said she would go to them on Christmas Day, and now they are struggling when DM can come and stay with them as they also want the other grandparents to come and stay for new year.
AIBU to put my foot down and say that, actually, this time, I'm not changing my plans for them and we will be at DM's for Christmas?