To be a bit upset for DS that his cousin left his 13th birthday 'gathering' after an hour.....

[sensesworkingovertime]

....to spend the evening with friends she sees every week and go to a 17th party/meal?

I get that she's 16 going on 17 and wants to be with her mates but there are two main sores points for me:-

1. she didn't say anything along the lines of 'bye have a nice birhday' to her cousin (DS) on leaving, she said nothing to him and made a bee line to her friend at the door (should explain here that family was gathered at grandmas house).

2. Last year myself, DH and DCs were given about 2 hours notice on an invite to said cousins 16th birthday afternoon buffet (as far as I knew she was just having a party the previous evening with her mates). I had already made arrangements to see a friend which I then was forced to cancel as did not want to upset niece, my brother and SIL.

Basically, you know as a mum how much you feel any upset for your kids and I know he would have been looking forward so much to having her there. Just a bit upset really. Thanks for listening.

Did she tell you beforehand that she'd only be able to stay an hour?

She's probably very close to the person whose 17th party it is if she sees them every week. She showed her face and stuck around for an hour (which for a 17y/o at a 13th birthday is pretty good) therefore should not have to miss a close friends party to stay longer.

I can imagine that if someone posted 'AIBU to let my DD leave her cousins 13th birthday early go to her friends 17th' many people would say YANBU

It was however rude not to say goodbye properly

yabu, are you generally pissed off with your brother and his wife?

yabu

YANBU to think she should have said something to your son, YANBU to be annoyed with your bro and sister in-law for giving you such short notice for your niece's party. YABU, to expect her to stay at a younger cousin's family party when her, possibly very close, friend is having a birthday that she could well have been invited to way before your son's. Or at least known about and accepted an invite to.

Best bet is to hope her manners kick in, or that she was just generally being forgetfuk, and to get yourself all some courage to say 'no' and 'i'm busy so i'll stay only an hour' when pushed into something at a last minute rather then breaking plans and rearrange your lives for people that can't be arsed to organise. It's also impolite to expect people to just drop everything and come to something-however special- so if they expected it, rather then just hoped, they were being impolite and U. But that's your issue with bro/sis in law, not your niece.