@Bold: I feel that Bano aapa feels that Zain's mother is already punished for her mistakes and by telling her truth to Zain they might make him feel bad about what his mother did. As she had already died, they don't want Zain to feel any resentment towards his dead mother. In yesterday's episode also there was a dialogue like that if someone got punished for the mistakes we could cover it up.
What you said makes perfect sense but here zain is unnecessary putting all the blame on iman so I feel he shouldn't be kept in the dark but I got where bano apa is coming from...

