The Viridescent Tiara (Siya)

The frigid, magical Latia Duchy is the setting of a novel Lan wrote as a teen. But after waking up as his villainous stepsister, the remorseful author strives to atone for her tragic male lead's ending. Until Eustaph can come of age, Lan steps up as the temporary duke to improve his beloved dukedom. But Eustaph is a dangerously capable future duke–as an outsider with no Latia blood, Lan may only be in his way... unless common goals can bring the two together.

(Source: Tapas Media)