From characters POV

I think we are playing the blame game as we are looking at the trio - Dev, Sona and Ishwari from audience point of view. If you view the behavior from character's point of view, not each of them is aware of other's shortcomings or weakness which usually happens in real life too. We can judge them as we are able to witness what each one is going through, the depth of emotions and how each one is interacting with each other.

Dev - Poor guy!! He is made to believe by his mother that her happiness is in his. He doesn't know her insecurities and the way his love life is affecting her as a mother. He always saw Ishwari as a woman who worked day and night to give a better life to her kids. He doesn't never have second doubts about her or having grey shades in her personality.

Ishwari - She is not aware of how much Sona means to Dev. How she changed him as a person or the depth of their love. All of this happened so sudden that she could not handle that Dev can be so emotionally attached to someone other than her. She feels threatened as the son who never lied to her took so much time to share his biggest happiness with her. Again Dev suddenly giving so much importance to Sona in front of her is not what she is used to seeing.

Sona - She is the least complicated one out of them. All she wants is Dev and Ishwari to accept her. She is constantly being taunted by RR and Ishwari and evil eyed by Vicky. But she does not share everything with Dev. If she was most vocal and told Dev each and everything that is being said to her, things would have been different.