My OH's parents live in Cornwall and as they will be away for a couple of weeks, they have kindly offered us their beautiful house if my OH and I fancy going, with or without another couple. We have invited my aunt and uncle who we have a nice relationship with and whose company we enjoy. My aunt however won't come because it's my cousin's birthday (she will turn 54 - she has a husband and one kid still at home) and my aunt wants to see her on that day (which would be right in the middle of our break so not possible for her and my uncle to join us the following day as it'd make not sense to travel that far for 48 hours). No formal celebrations - she just feels that she can't be away on her daughter's birthday. They live fairly close to each other and see each other at least a couple of times a week.
Is it odd that my auntie thinks that the idea of her not seeing her daughter on her birthday is mad? Do you see your parents on your birthday each year? Or your adult children even if nothing's being 'organised' to mark the day? AIBU to think it's very strange behaviour on my aunt's part?