Well, I think the entire forum wants to bash up Prachi for not opening her mouth and revealing the truth atleast when she was alone with Milsi at the chawl. I too felt that she should have done so then if not much earlier. But this is my interpretation of why she kept mum.
When Prachi recollected Milsi's conversation with her telling her that he could not bear to go through another heart break, she was trully terrified what he would have done to himself. That was why she panicked and went in search of him. (By the way as to why she did not look for him in the night, the explanation could be that she wanted to leave him alone till he got over the shock. She knew she had caused him pain but felt that given time that pain would subside. So did not want to disturb him in his guest room where she must have presumed he slept.) She then recollects another conversation with her where he tells her that he goes to the chawl whenever he is very happy or very sad and finds him there.
Had Milsi reacted with tears and pain I think she would have admitted the truth. If there is one thing she cannot bear that is to see him in pain. But instead she saw him angry, frustrated and suspicious. This was a face she was used to seeing. She felt he would be better off venting his anger at her than if he had been told the truth. The truth she felt would break his heart because of his love for Suki. She always thought that his love for Suki was much more important to him than his love for her.
Now dont ask me why she thought so when he had clearly expressed his love for her and told her that her happiness means more to him than either his family's and her's. Even his actions showed that Prachi is the most important person in his life. Why else would he forget his sister's bidai and be caught up in establishing his relationship with her. Even on Ritwik's birthday he was not around to see her off on her honeymoon because he was busy getting his wife to agree to start a family with him. But the dimwit thought otherwise. Do you recollect the waterfall scene where she says that she treated his sister as her own and wanted her happiness because her unhappiness will inflict pain on him? So in her convoluted mind all she can think of was Milsi's heartbreak if Suki's raaz is revealed.
As I said if she had seen him still heartbroken even after spending the entire night in solitude she would have gone ahead and told him the truth.She would have realised that the lie she said was hurting him more than the truth could ever hurt.
Unfortunately she does not realise that Milsi expresses his pain through anger. The more he hurts the more he tries to relieve it by converting it into anger. Hence his whiplash at her was his way of telling her how much she hurt him. She, on the other hand, was only going by his words and feels that he hates her (which though unbearable to her was better than seeing him in pain and heartbroken). That is why she finally tells him that she can bear anything from him. He can hate her and take away all her rights but she does not want to see him punish himself.
Now I am not passing a judgment as to whether she is right or not. But I guess this is the way Prachi saw and reacted to the situation.
In my opinion both Chi and Mi are wrong in their own way - Chi for hiding the truth from a husband who loves her dearly (especially a truth that concerns his sister which he has a right to know); Mi for still failing to trust a woman who has given her everything to him and has proved him wrong time and again by her acts. Maybe Chi right now is a little moer in the wrong than Mi though because Ritwik's future has also become a big question mark as a result of her lie.