Migawari no Hanayome wa, Bukiyou na Henkyou Haku ni Dekiaisareru

Clarisse, second daughter of Viscount Fahrenheit, was treated cruelly by her family. When she became nineteen, her father arranged a marriage for her...to marry Margrave Gutenberg, who asked the King for the "Jewel of Fahrenheit" as his bride, as a reward to his victories in the war. But the "Jewel" was her sister, Matilda. Clarisse was a substitute bride. Clarisse was almost happy that she could get out of her miserable life at home, but also afraid that the margrave would know she was a substitute. It was a surprise when she found out that he was rather a thoughtful and handsome man. She started to enjoy spending days with him, and it seemed like he feels the same way. Then came the day that her family secret was revealed...! (Source: MangaPlaza)