From Wingtip Cafe:

In Heian era, a noble chief councillor of states had 2 children who wished they were opposite gender. Lady Kira is strong, willful girl, while Lord Kira is weak, beautiful boy. Because of their characters, their father raised Lady kira as a boy and Lord Kira as a girl.

The manga is based on a novel from late Heian era called “Torikaebaya Monogatari”.