I know I'm being unreasonable but..
MIL is alone (divorced) and always comes to us. She mentions from August how she can't go to other family- too noisy, too far etc. I invited her several weeks ago to come on Christmas Day late morning as we won't be eating or unwrapping until the evening and have just received this email:
"Hi DSIL has phoned and told me that you are all going to them on Christmas Eve and would I like to go too, Is it convenient for you to have me to come up on Christmas Eve and go to DSIB's with you lot and then come back to yours overnight ?
I could go to DBIL on CE and stay there but there is a problem on C morning as DFIL has been asked to them and DBIL can't cope with Dad and I being there together, you know I am not good at driving from DBIL to you so hope I can come CE to you.
Oh families and Christmas! I would have gone to (her sister's) but they have (unlikely but insurmountable other engagement)."
Coming to us on Christmas Eve will completely change the dynamic. She'll be here late morning and that means that 'Show Christmas' rather than 'happy but messy preparation' starts then. I feel railroaded, particularly by that end bit- 'families and Christmas!'... I invited her!