Meenu ,
If we the audience extract ourselves from what we want the story and look at it from the point of view of characters and the place where they are one would find justification in their behavior.
Dadi for instance. She is a women who does not have much of a say in the household. She is abused by her husband and her DIL and generally keeps silent and out of their way . Helpless in her state she is resigned to her place in life. She has a very deep bond with her grandchildren specially Viplav . She listens to him and is always happy with his choices. As Viplav says "In the entire household its only you who thinks about me " .
Dadi does not stand up or raise a voice nor her opinion. She has been watching Viplavs turmoil and is empathetic. With Shallu's marriage breaking off and her trying to kill herself she for once takes action. She calls upon the age old - Sacrifice card - on Dhani knowing well that this is the only way she can restore some peace in the house. It will hurt Viplav for a while but eventually with Dhani going far away in time he will forget about her. Dadi from her very small point of view is trying to save the family . And I think she is within her character to do so . Its the only thing she knows - to give up your happiness to keep the peace.
She is witnessing a defiant Viplav and knows that he will not adhere to her wishes and Dhani is her only bet . Its not that she does not care about her grandson ..she thinks this is the ONLY way to bring back peace and happy.
Unfortunately Viplav will never get to know about who forced Dhani to change her stand. Neither will she tell him.
Its for Viplav & Dhani to stick to the promises they have kept to each other .