For the last few years me and my brother have split having my Mum and Step Dad. I'd imagine it's a normal type thing. One year they come to us for dinner, next year they go to my brother and his OH. It worked well because my GF has 2 DC from a previous and on the years we don't have my Mum we have gfs Dc for dinner, on years we have my mu the DC go to their dads.
Now this year we're due to have the DC and my Mum goes to DB.
But this year DBs OH has decided they're going to her daughters again instead and my Mum and step dad arent invited. This leaves my Mum and Step Dad sitting at home on Xmas day for dinner unless we invite them. (Which I inevitably will)
I don't think I'm being unreasonable to be annoyed, but I don't think I should say anything.