Koukyuu no Dansouhi: Nazotoki Musume wa Koutei no Okiniiri

Suiran, a girl raised in the mountains and skilled in martial arts, enters the inner palace as an imperial consort, taking the place of her sickly twin sister to bear the Emperor's child. Emperor Gishū, who possesses a beauty that seems otherworldly, appears interested in the brave and strong-armed Suiran. After being labeled an eccentric by Gishū, Suiran is tasked with investigating the true identity of the "Ghost" that appears in the inner palace...

This is a Chinese-style inner palace fantasy serialized on Futabasha's manga website, Web Action.