Kaiki Gensou Yume Monogatari Kaijuu Kidan Oboro Juuya

The nation of Itaru is shielded by four legendary beasts
representing the sky, the earth, the day and the night.
Here people are given birth as a girl with no exceptions,
and later choose to become either man or a woman.

This is a story of siblings who shoulder a fate to see the truth...

[From DLsite]