Complex sentence: Placidia was the second daughter of Valentinian III and Licinia Eudoxia, younger sister of Eudocia, who became the wife of Huneric, son of Gaiseric, king of the Vandals.
Split into simpler sentences: Placidia was the second daughter of Valentinian III and Licinia Eudoxia. Her older sister was Eudocia, who became the wife of Huneric, son of Gaiseric, king of the Vandals.