Everyone and anyone is your aunt.
I asked Mummy once, ‘who is Auntie Dorothy to us? How is she related?’
‘She’s not related to us. She’s a close friend’ she replies.
‘Then why do we call her Auntie Dorothy and not Mrs.Teoh?’
‘It’s a sign of respect, Melissa.’
Over here, you address anyone older than you as Auntie and Uncle. They may not be related to you by blood or marriage, they could be friends of your parents, or even the friendly laundry lady. The man who drives a school bus is Uncle Bus, and you say ‘Thank you, Auntie’ to the lady who serves you noodles.
It’s a common and acceptable habit in Asian countries to acknowledge a person who is notably older than you as Auntie, and not Mrs. so-and-so because the formality is seen as disrespectful.