Another realization comes to me...
Naintara does know what nivi is doing!
Cause she did not say, Nivi why are you loving this boy , j and cc son etc . etc.
All through out the conversation she said, do not put your life at stake cause she knew what exactly nivi was doing. Sacrificing her life for her sister.
But she failed to realize that during that process, nivi has thrown sid away from her heart and now, will marry viren, not out of love but out of respect (as many pointed out) as her own words indicated... Main bohot agay bar chuki hun.
Its almost impossible for anyone, who is soo indebted in gratitude to ever turn even the smallest favor away, so how can , nivi turn this away?
Naintara is angry because, she does not want any relations with teh soods and she always supported Keshav, he was insulted at the hands of sood thus his daughter being the DIL .. is not possible in her eyes.
She remains the same in her stand, when she told viren,
Nivi thumhari manzil ho hi nahi sakthi.
Because he was a sood.
Nothing to do with Viren. So once she sees nivi actually going against her feeling , she gets upset and slaps her.
But I am sure, she realizes that, nivi has not accepted viren out of love. That was already clear in her mind. She just dosen't want nivi to ruin her life in her POV