Shukujo no Kagami Yamemashita. Toki wo Gyakkou shita Koushaku Reijou wa, Wagamama na Imouto ni Furimawasarenai You Seikaku Waruku Ikinobimasu!

Betrayed and killed by her cunning half-sister Muriel, noble lady Christina meets an untimely and bitter end—or so she thought. When she suddenly wakes up three years in the past, she finds herself with a second chance at life and vengeance. This time, she refuses to be the ideal noblewoman everyone expects. Teaming up with her fiancé, the sharp-witted Second Prince Iril, who is investigating strange incidents along the border, Christina prepares to fight back. She’s done playing the perfect lady—this time, she’ll survive with a wicked smile.

(Source: Futabasha, translated)