I would call 'Didi' to any woman ('Bhaiya' to any man) older than me (not in the workplace, not in school)... Maybe on the bus, in a vegetable market.
I would call 'Bahan' or 'Bahan Ji' to a woman who I think is of my age. (again not in the workplace or in school). And of course, I wouldn't call a girl 'Bahan Ji', if I like (love) her.
Thanks for asking Kristy, 'Bahan Ji'!!