Responses

He didn't or He did (idrk this part). All I know is that Keira's mom like the Duke and admire him. Also the one who instructed to assasinate the 1st wife was the Patriach of Keira's maternal family (Grandpa). I just read it too so I don't think it's accurate but atleast we get a glimpse of what's happening.
Spoiler from someone I came across somewhere:
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It is true that Cosette is the Duke's daughter from her 1st wife. Furing the time when his wife was chased out of the Parvis family she was pregnant. When the marques (Keira's maternal family) knows about it they pay to assassinate them. During that process Duje's lover (1st wife) made a deal with the demon in order to revive the baby Cosette but instead of granting that wish the demon coveted Cosette's body and used it for her own. That is the reason why the current Cosette looks like the Duke.
If you're gonna ask if Keira is the only Parvis lady. The answer is YES since the real Cosette is dead.
The real Cosette is born of mutual love from her Parents.