So, you grew up calling her "auntie" as a term of respect, because she was your elder, not because she is a blood relative?
Is she married? Is she is widowed?
This is all very difficult, because I think it matters a lot if you are in rural India or a big city and her family circumstances. You don't want her family to take offense and beat you up!
Maybe you could start by saying that you used to call her "Auntie" when you were a boy, but now you would like to be friends as two grown ups and if you could call her by her first name.