Complex sentence: Elizabeth was the only daughter of Stephen, Duke of Slavonia, a younger son of the Hungarian king Charles I. Her mother was Margaret of Bavaria, and her only known sibling John, Duke of Slavonia (1354--1360).
Split into simpler sentences: Elizabeth was the only daughter of Stephen, Duke of Slavonia, and Margaret of Bavaria. Her only known sibling was John, Duke of Slavonia (1354--1360).