The Clan's Heir is a Trans Woman(2013)

Overview:

The head of the Tatsumi family falls ill and dies after requesting that his son, who disappeared 15 years ago, takes over. The Tatsumi family, including the young head Ryu, decides to search for his son's whereabouts in order to keep his will. They finally find his son and learn he is now a woman. Meanwhile, the hostile Honjoukai are plotting to steal the Tatsumi family's territory.