Spoiler
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The Duchess treats ML like that because he's the result of an unpleasant memory with the Duke. The Duke was drunk and thought the Duchess was his past lover, in the end the couple slept together and then the Duchess had Karhan from that night. So yeah, That's the reason why she treats him like that. Though in the end she will beg/ask for his forgiveness. Worry not, Karhan won't forgive her.
What about the brother?
The brother is actually not a good kid like the Duchess said. He's a spawn of satan but all his wrongdoings were dumped to Karhan while his 'good deeds' were actually Karhan's doings. Like, there's one time there's a dead bird body lying on the duchess' room and she thought it was Karhan's doing while it was actually the brother. And the other time there's a handful of wildflowers on the windowsill of the Duchess' room. It was actually Karhan, but she thought it was the brother and the brother is taking the credits by saying "he bought it for her."
In the end, the Duchess finally realizes that all the bad things were her first son that she proud of all along and not Karhan. But it was already too late to ask for forgiveness because Karhan already have his new family which is the Blodin family.
Sorry for my bad english pookie :(
thankyou so much for the spoilers. Really glad the author diidn't force a reconciliation between the abusive mother and karhan
Aww not a problem pookie ヾ(❀╹◡╹)ﾉ~
Any spoilers why she treats her own son like that?