In my opinion a person is bad/good is due to his choices and not due to upbringing. Ishana's choice and upbringing both might have made her a conwoman or may be its just the choice. Upbringing may not have affected her in anyways, it's just her choice.
I totally believe in choices being the reason for what we are, at every turn of life we get choices to pick and what we chose shapes us. Upbringing does have an influence but to imply that upbringing in our life is also a choice. That's why there exist rigtheous parents having evil children and evil one's having righteous children in the society.
Let's consider the upbringing of fictional characters of this show itself;
Shivaay - keeping aside Pinky and Shakti if we consider his daadi as upbringing him and implanting values then he is still diffrent from her in many ways, she has thought him not to judge people by their status but yet he does it, because it's his choice.
Omkara - Tej's son, always surrounded by Shivaay yet he has diffrent traits, views than them. Well, we can say he is like his mother then too it's his choice that makes him to be like his mother and not like his father.
Anika - no upbringing, an orphan yet has righteousness because it's her choice.
Tej & Roop - Kalyani daadi and her husband are both righteous people going by daadi's actions and her description of daadaji but the children are far from righteousness, is it the upbringing? It's the choices they made once they grew up.
Upbringing may or may not be the reason to shape ones persona but choice definetly is. Ishana has just made a choice out of so many options out there, just like Shivay chose to throw water to bring Anika back to senses out of so many other options 😆