I can already tell the story beats.
She’s of course the true daughter.
The sister actually isn’t the daughter of the king.
The reason she turned ibto a child is because the curse affects those with royal blood in that manner.
The main guy will learn that she’s royal blood before the MC does.
When this all finally resolves, the king will do a shitshow of an apology attempt.
also
King will get dethroned and Duke/ML will marry FL and become new sovereigns
"Sister Princess" will get humiliated along with Prince fiance (after much abuse and harassment over the course of the story toward FL/ML), and will either get the exile/complete social death treatment, OR the FL will "magnanimously forgive and pardon them" to show just how nice and special and perfect she is
ML will be very flustered over "falling in love with a child" even though she's of-age in the technical sense
"Magical Plants" will provide some super Deus Ex Machina solution to some convoluted problem befalling the kingdom, and everyone in the palace will freak out and cry doom before the FL shows up and conveniently solves everything instantly