My brother currently has a gf who he has been with for a few years now. He is twice divorced and there have been many gfs over the years.
The current gf decided when my ds was born that she should be called 'Aunty' which has irritated me from the beginning because I hardly even know her and they are not married.
My Dad died recently and my brother has been a real let down and she hasn't done anything to help so I'm especially rebelling against her decision for my son to call her Aunty.
YABU - it's up to her what she should be called
YANBU - it's up to the parent to decide who has the honour of being called Aunty unless they are actually an Aunty!!!