I would write this entire post in shouty caps. I know it would make this unreadable though so here is the update:
FI got an EMAIL THIS MORNING from FBIL. It says:
Hi – I’m going to send back the RSVP today – it was in the pile of bills… XXXXXX and XXXXXX want to go to the reception; we will most likely have XXXXXXXX (the baby sitter) pick them up around 7:30 or 8:00.. is that cool?
I LOST IT. Absolutely went nuts in my office on the phone with my FI. Yes, please darling FBIL, have the kids come and then leave in the middle of dinner. Totally fine! It's your day! Forget about your brother and new sister in law's wishes. NO PROBLEM.
FI is telling him again that children aren't invited.
Vent over. Going to find a midol and some chocolate.