Shiitagerareta Seijo Kouho wa, Ringoku no Yandere Koushaku ni Dekiai Sareru

Amelia of the Elkin Ducal House was engaged to Crown Prince Yushi as a candidate for sainthood. Her achievements in healing soldiers were concealed, and the one chosen as sainthood was her stepsister Flordia, who was supposed to be inferior in magical power. Having lost both her fiancé and the position of sainthood, Amelia is ordered by her father, Duke Elkin, who has returned from inspecting his territory, to enter into a political marriage with Teddy of the neighboring Hamilton Ducal House.
https://www.cmoa.jp/title/295323/