How do they not find out that akito was a girl at the beginning?

Answer 1

In the anime series, Akito isn't a woman. But in the manga they find out it'll be a girl, but Akito's mother says she'll have an abortion if they don't raise her as a boy. Because she doesn't want any other woman getting attention other than her. So Akito is raised as a boy, and Hatori, Shigure, Ayame and Kureno all know Akito is a girl because they were there when she was born. But they keep it a secret. They went so long like that that no one says otherwise. Akito was raised like a boy, dressed like a boy, and grew to have a very flat chest so no one really noticed.

Answer 2

I don't believe Hatusharu knew that Akito was a girl. (I don't recall in any of the Fruits Basket Volumes that he even knew Akito was a girl). But I do know that Kureno (Rooster in the Zodiac) knew. And I believe that Shigure, Hatori, and Ayame possibly know Akito is a female.

Answer 3

Akito is able to hide the fact that she is a girl by dressing in male clothing and having a strong male presence. Additionally, the Sohma family is secretive and protective of Akito, making it easier to maintain the misconception that she is male.