Luciana is not Ariana's biological daughter
Luciana looks and acts very different from Ariana, and it's never revealed who her father was. After all, the king Ariana married would have been too old to produce a child with her (and three days of marriage is not exactly much time to do so in the first place). Ariana treats Luciana more like a ward than an actual daughter, and considering Ariana's behavior around others, it's imaginable that she took in Luciana and raised her to be a future queen.- Stronger enforced yet by Ariana singing the line "But when I heard that good King Peter had a son, I had a daughter rightaway!" Even if she could have gotten someone to impregnate her in short time, she could never have been sure that she'd have a daughter which she could wed off to Peter's son. An adopted girl gives her certainty and spares her the scandal of being a queen carrying and raising a bastard.
