Mao Marimo

Mao lives with his three sisters in a village full of the presence of gods. On the day of the village festival in his first year of high school, he was given a divine destiny: to become a girl. While Mao is happy and accepting of his new womanhood, the other people in his life have more trouble coping. His aggressive twin sister Akoya (nicknamed Ako), who used to tease him for being a feminine crybaby, seems to blame herself for his transformation. Both Mao's twin sister and his closest male friend from school are going to have a hard time adjusting the way they think of this new female Mao!

(Source: MangaHelpers)