Maybe they could reform the daughter? Shes still a kid after all, pretty sure shes just mimicking her mom
Maybe she'll live in exile in another country but nope I don't see her entering the family. In these stories, she's already set for doom.
Which is really a shame because it's clear that she's being manipulated by her mother. Not only does she believe that she really does have spirit energy, and is convinced the MC stole it from her(she's too young to understand that it's not only impossible, but a newborn infant, that even the father and brother weren't able to get near, would somehow steal ALL of "her" spirit energy), but her mother is clearly emotionally abusing her into believing that nobody could love her even with that spirit energy. So she needs to be as ruthless and manipulative as her.
But oof if only the Duke & King could hook up. Then it's one big happy family!
(Well after they get rid of the Empress & her daughter)