Dh has 2 dcs from a previous partner, they stay over every other weekend.
On saturday my ils have invited us round for dinner, my dsd has just rung dh to ask if she can sleep over a friends house on sat night (not a party), he said yes, im abit pissed off tbh because he works most weekends so he doesnt really see them much nor do his parents.
I would not mind so much but we have been invited out that night.
If IL's don't get to see her very often and DH knew you had cancelled other plans so you could all go to his parents house for dinner (not something that seems to happen often) then I would be pissed off with him for sure.
I'd be giving him 2 options
a) Ring his daughter and tell her that he's sorry, but he'd forgotten that you have plans with her grandparents, so no sleep over.
b) Let her go, reschedule dinner with IL's and go wherever you were meant to be going (take other DSD or babysitter - whatever you wwould have done).
I can see both sides tbh. At 12 sleep overs and things are important to kids. Yes, so are relationships with the wider family BUT at 12 I know I would rather have been at a friends than at my gp's and if I had been foced to go, I probably would have sulked the whole time and risked spoiling it for everyone else.
It's hard to say without knowing more - if you have the kids on alternate week ends, why do they not see the gp's very often?
If it is bothering you though, perhaps you and dp could have an agreement that if the dc's call to ask for something on their week ends with you, he won't give an answer there and then but you and he will discuss it and call back?
I have step kids and I know it's hard when you don't see them much and then they have an activity which falls during their time with you. We have the same dilemma sometimes so I'm not necessarily saying yabu, just that I can see both sides.