Himegoto wa Hanazono

Tanaka Ai is a 12-year-old boy who is the youngest of his 7-brother family. After being rescued from bullies by the neighborhood 'yankees' (delinquents/gangsters), it has been his dream to become one of them.

He's about to start middle school with his friend Suzuki You and his (older) twin brother Koi, when something very strange happens. Ai transforms into a girl overnight! His older brothers seem to think this is nothing out of the ordinary, but Ai is devastated.

As they start school together, with Ai in his new female school uniform, Ai decides not to give up on his dream. If he tries his hardest, maybe he can become a badass lady gangster!

(Source: MangaHelpers)