Imouto no Migawari de Totsuida Hazu ga, Douyara Watashi ga Shin no Seijo Datta you desu

Christina, who grew up in a family that mistreated her. Her sister, the saint, was supposed to be married into the empire, but Christina becomes a substitute and is married off in her sister's place instead due to the former's selfishness. Christina is to be married to Emperor Maximilian, who is famous for hating "saints"!
She is also sent to the frontier of the empire, but Christina is overjoyed at her newfound freedom! She fully demonstrates her knowledge of agriculture from her previous life and her magic hidden in her homeland, and reforms her surroundings to her liking.