Misunderstanding Marriage

Mireille, the princess of the small and struggling nation of Glires, suddenly finds herself the subject of a marriage proposal from Cain, the king of the great nation of Drake. However, it quickly becomes clear that Cain's true object of affection is her younger sister, and the proposal was made to the wrong person. Despite this revelation, Mireille is forced by her father to take her sister's place and marry Cain as a stand-in bride. Prepared to bear the guilt of this deception, Mireille braces herself for a cold and loveless life. Yet, to her surprise, Cain cherishes her deeply as his bride. Why does he continue to treasure her so sincerely despite the truth? (Source: Overlap, translated)