Raj marries Preet because he loves her. He actually believes that Preet couldn't have murdered Raman.
Harjeet gets to know that Raj is his son when he sees Amba at the temple. Subsequently, it is revealed to everyone that Mannu and Preet are one the same.
Mohini as usual tries to play politics. She is of the opinion that she can take revenge from Mannu aka Preet if the enemy is at home.
She convinces Harjeet to bring Raj and Preet home.
Mohini and Harjeet brainwash Raj into believing that Preet is the one actually behind the murder. And then the MU happens.
Or
As I said before, Raj wants to protect Rohan and Preet. At the same time, he wants to drive Mo and co. out of the house. So he pretends to hate Preet to be accepted by his father and gain entry to his house.
But what actually happened was dumb to the core! Absolutely senseless!