Manga Soredemo Sekai wa Utsukushii

Nike, the fourth princess of the Rain Dukedom and one who holds the power to call forth the rain, travels to the Sun Kingdom to marry King Livius on behalf of her country after losing a game of rock, paper, scissors to her three older sisters. Upon arriving at the Sun Kingdom, she discovers that the Livius, who conquered the world in only three years after his ascendance to the throne, is only eleven years old!