As much as I find Jeannette a weak character - not in the sense of character, but in the sense of development and depth - I don't really mind at all: she's a child thirsty for affection from a family of her own, and I don't think anyone has ever bothered to teach her how to behave in such situations (but also how to behave against intrigue in general, since she's always been kept in reserve as an ace up her sleeve and puppet to be commanded to achieve other goals).
Also, i wouldn't mind if she was actually considered the younger sister of Athy, adopted by Claude.
Although a tide of Isekai and manga in general teach us otherwise, a man can safely have more than one daughter. And incredibly he can love all his daughters in the same way, without idolizing one of them and treating each other like a servant or worse...