Complex sentence: He was the only son of Galla Placidia, the younger half-sister of the western emperor Honorius, and Flavius Constantius, who was at the time Patrician and the power behind the throne.
Split into simpler sentences: He was the only son of Galla Placidia and Flavius Constantius. The former was the younger half-sister of the western emperor Honorius, and the latter was at the time Patrician and the power behind the throne.