Gunshin no Hanayome

Sandwiched between an accomplished older sister and a beautiful younger sister, Sakura has lived an unremarkable life. Until the day when she is suddenly abducted to be the bride of Kai, the prince known as the "God of War." However, it is not for love, but simply because she has the power to sheath the "Sword of Demon-Slaying." Sakura is kept like a bird in a cage, but will her intelligence and kindness bring about a change in the cold-hearted Kai…? (Source: Kadokawa, translated)