Was it really a "big family celebration" or did it happen that one of the shows was on your mum's birthday as a happy coincidence so they went on that day as an extra?
Tickets were like gold dust, and there was a maximum of four per household. Mum, Dad, brother, his partner.
If one person booked they would have been fighting to get ANY ticket, and couldn't have got you one anyway - unless you went in place of someone else.
My gut feeling is: parents and brother/partner are Oasis fans. When tour was announced they said "Let's go."
When tickets came out it was "Oh, look, it's your birthday! Cool!"
Limit was four, booked four for the four people who like them.
"Am I not invited?" "It's, Weds, you wouldn't be able to come, and you don't even like them."
This has now, in the Op's head, been turned into a "big family celebration" when the reality is that it happens to be on her mum's birthday.