Migawari no Hanayome wa, Bukiyou na Henkyou Haku ni Dekiaisareru

Clarisse, a viscountess living in the royal capital, has always been compared to her beautiful elder sister, who is known as “the jewel of Farenheit.” She is forced to lived in the shadows, pressured by her family to keep a low profile. When Clarisse is nineteen, she marries Siegfried Gutenberg, the “man-eating frontierman,” as a reward for his success in war, in place of her sister. It turns out that the frontier count is a stoic, but caring and surprisingly charming man who treated Clarice kindly. Clarisse, however, feels guilty about being a “substitute bride,” and is afraid to entrust her heart to him. Then news of her sister's wedding arrives from the capital, and they decide to visit the capital for the occasion. But Clarisse's family hides a big secret, and the circumstances of her marriage are very different from what she expected! In this royal love fantasy, an unfortunate lady finds happiness with the love of her life on the remote frontier!

(Source: Kadokawa, translated)