Complex sentence: After his father Kōnin became emperor, Kammu's half brother was appointed to the rank of crown prince; but instead of his half brother, it was Kammu who was later named to succeed their father.
Split into simpler sentences: After his father Kōnin became emperor, Kanmu's half brother was appointed to the rank of crown prince. But instead of his half brother, it was Kanmu who was later named to succeed their father.