I'm a bit more clear in my head now. So I'll give my broad reasoning.
It has to be before. That's part of the journey. Mohan falling for her and then her finding out... eehhh. Whereas Mohan having a bit of of good relationship with her, her finding out how she deals with it all, how he answers to her finding out, and how he falls in love with her as she sticks by him and helps him through his journey.
That's what I want to watch. I also think (hope) that whatever Kastur finds out, at first it will be her finding out, while the family remain blissfully unaware. That adds more conflict and depth to the story.
Plus they can't have everyone find out at once for longevity purposes as well.