I notice a lot of people feel that Dev and his family were the only ones at fault.
All the stuff about Dev's family is correct no doubt.
However, Sona immediately in her mind separated herself, her family from Dev's. I think that's why Dev's mom was getting so upset.
A lot of families fight especially in-laws in new marriages, it's the most sensitive situation and time.
What I saw in Sona was a side that no one saw before in the family. Dev never separated himself even after the whole incident, because he was so concerned about how Sonakshi and her family would pay the huge interest rate and kept begging her to take the money back. It's because of Soma's abandonment towards him and his union, Dev was clearly upset.
At the end of the day, we can clearly see some ego in Sona. Dev did not slap her father with intent, it was clearly a mistake. Sona should have acknowledge. As Dev's wife, she should have supported this found another way of handling the money matter.