I think he has called her his patni when the entire house was against her during the trip fiasco. He also said Sai is their ek lauti bahu.
The thing is Virat shows that he considers her his wife. He is always caring for her. Being a typical jealous husband. He always wants her to accept him be it as a friend or a husband. He can't take his eyes off her.
Sai is the opposite. She tells everyone that he is her husband but she doesn't show any feelings apart that she doesn't like Pakhi and Virat together.

3