Yamauchi Naomi

Ruri, the hopeless daughter of the great General Fujiwara, vows to "be single forever" after the death of her first love. However, the General plans to force Ruri into marriage, whether she likes it or not. Not wanting a forced marriage, Ruri decides to find a nice boy to marry instead. Ruri's chosen candidate is Taka-akira, a boy from a prominent family and a close friend to Ruri's younger brother. With her problems apparently solved, Ruri and Taka-akira decide to get married. Unfortunately, things aren't as simple ans they seem and life has even more complications in store for the young couple... source: https://www.mangaupdates.com/series/w3vyjm3/nante-suteki-ni-japanesque-hitozumahen

This manga follows the tomboyish Ruri, the hopeless daughter of the great General Fujiwara. She commits to "being single forever" because she thinks there's no good man like her dead first love. The general plans on a chosen candidate to 'finish' Ruri off and then announcing the wedding afterwards. That's when Ruri decides she would better marry off some nice boy rather than be forced against her will. Taka-akira, a son of another great family and a close friend to Ruri's younger brother, has fallen in love with Ruri, and will become Ruri's husband. However, the wedding of the princess and young lord can't begin at ease it seems... source: https://www.mangaupdates.com/series/w14xhgw/nante-suteki-ni-japanesque

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”.