I just want to check I'm not alone in thinking the 'first come first serve' rule is the standard one 9/10?
MIL mentioned to me 2 weeks earlier than said event that her DSis was having a small gathering on a Sunday day time for her DD's 30th birthday and "if you're free" would DP, DS and me like to go. I replied that we would've love to but we already had plans for that day that we'd had for some time. (Not that it really matters but it was seeing my best friends who are DS's godparents to say bye to them before they go travelling for 6 months). MIL seemed to completely understand and that was the end of it.
Turns out MIL's DSis (DPs aunt) is 'very disappointed' that DP didn't go, and thinks he 'should've made the effort' as they're only a small family. DP text her that day to apologise for not being able to make it due to prior plans and he's had no response since. I'd said previously to DP that if he wanted to go we could try and go afterwards, or that if he wanted to go alone that would be fine but that DS and I would be sticking with our original plans. DP said it didn't matter. DP's aunt and her DD are not relatives we see very often or relatives that come to visit us or make much effort with our DS (this doesn't bother us, but just to give more context!)
This same aunt had also deleted me off Facebook along with another relative a few years ago, in protest of the fact that DP didn't go to a 21st birthday party of her nephew, because he came to my birthday celebrations instead. I didn't even know anything about this 21st party and definitely did not tell DP he couldn't go! We also lived together at the time!
AIBU to think it's really arrogant to expect people to cancel their existing plans to accommodate your (relatively) last minute ones? I've never been 'offended' by anybody who has told me upfront they couldn't make something I'd planned because they were already busy. Disappointed- sure, but isn't that life? Please tell me this isn't just me, and that that type of attitude is really entitled?