Kishin Kakka no Migawari Hanayome - Yotsugi ga Dekitara Rien desu

In place of her twin sister, Lady Tsukiko ends up in a marriage with Kishi, a young man who rose in the ranks as a lieutenant general. He's also known as the "Demon General" due to his ruthlessness. Since her childhood, Tsukiko has been sickly and deemed as a failure by those around her. After meeting the general only once, she was emotionally supported by his kindness. But now he tells her after she's provided him with an heir, they must get a divorce?!