yes...i was thinking along similar lines..especially when Dadi said something like the fact that she couldn't forget that woman's face.
i was thinking it could be khushi's mom.
But I'm going to say that they might have had an innocent love story. When met with opposition Khushi's mom probably said no and backed away.
But Arnav's dad was angry with his mother and was cold to his eventual wife...(anjali and arnav's mom). Maybe he never completely loved her as he couldn't forget his first love (which was very innocent by the way).
Arnav's mom was always emotional as she couldn't not get the same level of love and emotional connection.
then maybe khushi's mom and dad were in accident...and khushi's mom needed money for her husband's surgery (a husband who she now loved whole heartedly along with their little girl). The only person she could think of was her rich first love...she went to him begging for money to save her husband. he still loved her and then I don't know.
Maybe he was angry adn rude and bad that he said leave everyone adn come with me only...and she could have said no...and screamed when he tried to be misbehave with her...arnav's mom walks in and sees this and mistakes it for a mutual thing. She runs out of their and commits suicide as this for her was the cherry on the top of the icecream...and then maybe Arnav's father is cold and and doesn't get affected and instead still wants khushi's mom and then she runs away back to the hosptial and then witness her husband die and unbeknownst to everyone Khushi's mom was all along suffering from internal bleeding from her injuries and she also dies.
arnav's dad is angry and crazy that his first love is dead...adn doesn't give a hoot about arnav and anjali's mom and creates a ruckus at prayer meet and then disheartens the entire familiy. Dadi will never forget khusi's mom's face as it upsets and she blames khushi's mom for ruining her son. Nani knows that it wasn't that girls fault and perhaps she asked khushi's mom back in the day to back away...and khushi's mom in typical khushi like behaviour agreed instantly saying she could never seperate a son from his family and wanted know part of that.
Khushi's mom only came back that fateful day as she needed lots of money to save her husband. Khushi is left at the hospital crying and it's there that Shahi and Garima see her crying her eyes out and fall in love with a little girl who will eventually becomes their sanka devi.
That's it!!