When we were little we used to call my mums best friend Aunty Lou (we still do actually & im 36!!) but she has never once referred to us as her Nieces, because it’s bloody weird !! I think Aunty is used kind of like in the Asian communities, an older female, maternal figure in your life. I have a lot of love for my friends kids & I would hope they look at me as an “Aunty” type figure, but I would never lay claim to them as my nieces and nephews. But it’s typical BM - no boundaries, narcissistic behaviour. God she must be absolutely exhausting to deal with in real life.