Misunderstanding Marriage

Mireille, the princess of the small kingdom of Grilles, is an old maid when she suddenly receives a marriage proposal from King Cain of the great kingdom of Drake. It soon becomes clear that the woman he fell in love with at first sight is her younger sister, and that he has mistaken her for someone else. However, by order of her father, the king, she is forced to marry him in her sister's place. Mireille was prepared to bear the burden of her deception, but for some reason, she continues to be treated with care as a bride—!? (Source: Overlap, translated)