Futago no Kishi

This is a follow-up on the Princess Knight (Shojo Club version) work. In Twin Knight, the twin brother and sister, children of Sapphire, are the main characters.

After becoming the princess of Silver Land, Sapphire marries a man named Frantz of a neighboring country, and gives birth to the twins Prince Daisy and Princess Violetta. But because of the wicked designs of the Duchess Daria, Prince Daisy is abandoned in the forest.

Meanwhile, in the castle, Princess Violetta decides to play two parts: being herself and disguising herself as the Prince every other day to conceal the fact that he has disappeared. But her secret is discovered by Duke Daria, and the King, Queen Sapphire and Princess Violetta are confined in the northern tower.

Princess Violetta disguises herself as a boy and escapes with the help of a swan from the northern tower to look for her brother, Prince Daisy.

(Source: tezukaosamu.net)