Nah the Duchess knew that the Duke was drugged too. She doesnt blame him for that. She actually treats Olivia really well cuz its not the kid's fault. But seeing her husband act so evil and heartless and just the whole situation stresses her out so much. She repeatedly tells the Duke its not the kid's fault and to stop harassing her but he keeps blaming Olivia for everything, even the fact that the Duchess keeps fighting with the Duke , he somehow twists it that its Olivia's fault they are fighting. So the Duchess gives up. She gets super depressed and demands a divorce and leaves. And dies. Why does she have to die. She was the only person who treated the mc with kindness

I think ppl dont know this spoiler
The father did not have an affair. He was drugged and raped. The imperial family are the main villains. They threatened the Mc's mom to drug and sleep with the Duke. She refused ofcourse so they drugged her too and threatened her. So she slept with the Duke then ran away. She was a victim so you do pity her but then she turns around and does this shit. She knows the Duke hates her but brainwashes Olivia that if she was good, the Duke and his family would accept her. Even if she was dying , why would she send the fl to that hell house I dont know. She tells her to just keep bearing the torture since young so Olivia becomes a pushover.
Also the Duke is irredeemable trash. The absolute worst. Yes he was forced against his will. But he literally tortured a tiny little kid. He purposefully got her in the house to send her as crown princess knowing the royal family are evil, he wantedly appointed that specific nanny cuz he knew she would torture and abuse Olivia and starve her. It got so bad that the Duchess lost all love and hope for him watching his act like a villain.