I think I know!

Arnav's father must have been in love with Khushi's mother. Khushi's mother was probably already married to someone else with Khushi as their daughter. This could have been a one sided love or two sided. I'm thinking it was only Arnav's father that loved Khushi's mom. (much like Shyam was in love with Khushi). Arnav's mom might have found out about this and maybe committed suicide?

Too many people are predicting this. I think CVS will blow us up with a much bigger twist god knows what. I refused to believe these stuff, it's just too predictable and most people prediction is the same. 

It's IPKNND anyway anything is possible.