Responses
Well, that novel focuses more on the heroine more than anyone, so I don't think exposure of the Duke is more than necessary. Amelia died in that novel, so he went over the crazy and straight to homicidal. Also, the 2nd brother never appear in the novel, I think. Probably because Amelia wasn't there anymore.
It just dawned on me that many people in that world know how crazy her brothers are... How did the readers miss that!? Was the Author hiding it for the sake of the story or did they not even know?