Had a family BBQ last weekend for about 20 adults and 20 children. Sent out invites well in advance with RSVP request so I could plan food, drink and entertainment. Invited next door neighbours who said they weren't sure and would let me know. They never did. I gave them the benefit of the doubt thinking they were on holiday or something. However, discovered during the BBQ that they were at home and just never bothered turning up. Them not coming didnt' bother me that much - certainly didn't ruin a fab day but AIBU to be really annoyed at them not letting me know one way or another. They must have been sitting in their house listening to the noise of the kids playing. Is it just me or is this odd/plain rude. I know there are more important things to get cross about but I am boiling about this. Good manners cost nothing.
Their initial response sounds like they didn't want to come and didn't quite like to say so. I'd certainly have taken it to mean they would let me know if they did decide to come (and not bother otherwise). Are they friends? If not they probably thought you'd only invited them out of politeness and didn't really want them.
If they only live next door, why didn't you knock on the door and ask them if you weren't sure if they were coming or not?