if i had a lunchdate planned and got an invite to a "we have to do this as we're being moved etc" close family/friend wedding, i'd call the lunch friend and ask if we could reschedule due to the sudden wedding. and if i was having a lunch date its likely that person is a good enough friend that they'd understand and tell me not to worry about it.
none of the arguments anyone has put forward have convinced me that the grandma was anything but rude
By the same token you could also say that if MIL is close enough to be invited to daughter's birthday party, she's close enough that the family should understand if something 'more important' comes up and tell her not to worry about it. Not seeing the difference.
Also, you can't reschedule someone else's party, like you could your own lunch date. So, what was MIL to do in that situation?
if i'm having lunch with a friend and tell them that i'm missing a wedding, that was pulled together last minute, then all of my friends would tell me i was daft and should have rescheduled the lunch. a last minute wedding for emergency purposes trumps a chatty lunch in my circle.
i've told friends before that their emergency beats any plans we have so not to worry and to call me when things settle down and we'll catch up then, because thats what friends do.
one of my BF's is having a rough time with her father, terminal illness, and felt bad that she had to cancel coffee with me. i sent her flowers and told her i was thinking of her and if she needs anything to let me know & that we can have coffee anytime.
i also have had someone who was invited, 18 months prior, to my wedding, tell me 3 weeks out, that she's been invited on a trip that same weekend and so can't come to my wedding. we haven't spoken since.
the problem with this is that it was a last minute invite(3 weeks for that sort of party is last minute in my opinion) but it wasn't an emergency situation. Mil double booked herself and should have had the brains to tell aunt that she was already booked and would have to see them another time to celebrate.
I don't know that it always needs to be an 'emergency' though.
I mean, in the situation with the OP, it was the woman's sister
. The OP isn't close to the Aunt, but it's the MIL's sister. Obviously the MIL is close to her. So, who on earth does she upset in that situation? Her sister, or her granddaughter?
I do think sometimes people need to be mature enough to say "This is an awkward situation and I can see you're doing your best to accommodate everyone and not upset anyone".
I think I'd be pissed off about your wedding situation, though, if it was just a run of the mill trip that could have been taken at any time and depending on the depth of the friendship; that's gotta hurt. But again, I think a wedding trumps a 'trip'.
Case in point: My best friend of many years had 'other plans' on the day of my mother's funeral. She'd already arranged to go away with her boyfriend that weekend and said she couldn't postpone her plans because he'd booked the time off work. That stung. Technically she was doing the right thing per etiquette and I didn't hold it against her, but, man. A great example of why etiquette isn't always the 'right thing to do'.