Try to keep brief:
DH birthday on Sunday. New steak place opened up near us a few months ago and we said MIL would enjoy as she likes steak, none of us have yet been but I would love to go too, quite fancy place. Anyway we arranged for 3 of us to go out to celebrate birthday Saturday eve with my sister babysitting. MIL said it would be her treat. Sister can no longer babysit and MIL said that she and DH could still go.
i don’t know why but i feel like a bit annoyed. It’s somewhere we wouldn’t go to often and could rearrange when my sister could do but then again I also feel like maybe I ABU and should just say two of them to go?