I think its because Nivea was born out of wedlock I believe and the other children were born once he was married.
I think the similarity you saw mightve been a mistake from the artist. Nivea's mother is dead and she lived in the annex when she was alive. stayed in the annex while she went there as well so the marquis had the building removed with the excuse that they'll renovate it. Then Nivea told him to rebuild it as a sort of tomb for her mother. Nivea is treated like trash because the marquis hates her (I think because when she was born her mother died but it's been awhile so i don't remember what the deal was exactly).
Guys so I am hella confused. Nivea’s mom is a maid, right? And when they went into flashback mode, Nivea’s mom and the current wife of the Marquis looked similar (same mole, hair color, eye color). So I’m wondering why Nivea was treated like trash while the other two children that came out the same hole are treated differently? Or like, am I just wrong about Nivea’s mom looking similar to the current wife? And, if they aren’t the same person, shouldn’t Nivea’s mother be dead? Bc I think Nivea was telling the Marquis to let her mother live in the annex?