Long time lurker, short time poster.
My SIL has invited us to her DS 16th birthday meal, fair enough nice and thanks for the invite except it was my DS 18th last year, had a small family party as was his wish to which she/her family were invited, they didn't bother to come, not even a card, nothing. She wasn't busy/ill/didn't forget because she asked my other SIL about it the next day.
This Christmas nothing, not even a card but quite happily excepts cards/money for her 3 children the eldest of which is 27 and a DIL!
As not to drip my SIL has been one of my closest friends since we were tiny, she looked after my son when his nursery shut down, has known him all his life.
I married her brother but son is not his.
AIBU not to go? It isn't the lack of card/gift whatever it was the sheer lack of acknowledgement for my DS and our 40 year friendship, she didn't have to bring anything :(