As we have seen that Swetlana has wrinkles etc. of an old woman which she conceals by injecting the blood of FakeLana ( a new scientific way), so we concluded that actually this girl is old and is nearly of the age of Dadi, she was actually the GF/mistress of Dadaji aka PrithviRaj Oberoi and was also the mom of Mr. Kapoor (i.e. she is the dadi of Tia), this is the reason she knows about the chamber of secrets and even the areas in that arena which not even Omkara knows (as dadaji had made that room)
Tej got to know about this and he got angry (just like Omkara is now, remember he was not a jerk in his youth) so he avenged by destroying his father's mistress' son Mr. Kapoor who committed suicide
Mrs. Kapoor wanted to avenge this and she trained her daughters for this, but Swetlana (real one whose pic we have seen was not willing to do this, as she was good at heart)
So dadi Kapoor got a plastic surgery to look young and with a new face of Fakelana n also took the identity of her grand daughter Swetlana (since dadi Kapoor didn't have the qualification to become the secretary of Tej) and joined Tej as his secretary and slowly started attracting him. She wanted to show that the mistakes which Prithvi Oberoi had done those could also be done by his son and now grandson, Her aim is to become Mrs. Oberoi, irrespective of whom she marries,
So this is the raaz, what say