Paros no Ken

From ShoujoMagic:
There is a legend in the kingdom of Paros -- a legend of the Sword. It is said that if one who is unfit to rule should wield it, it will mean the ruin of the kingdom. As sole heir to the throne of Paros, Princess Erminia must accept the Sword and a husband when she comes of age on her next birthday. But the princess refuses to wed any man who is less of a man than she -- which excludes every man in the kingdom! Just when Erminia is ordered by her father the king to wed her foster brother Yurias, she has a fateful encounter with the unfortunate, beautiful maiden Fiona..!? "The Sword of Paros" is a miraculous romantic fantasy about defying convention, transcending gender and status, and knowing oneself.