Complex sentence: He had two half-brothers, Ptolemy Keraunos and Meleager, both of whom became kings of Macedonia (in 281 BCE and 279 BCE respectively), both died in the Gallic invasion of 280--279 BCE (see Brennus).
Split into simpler sentences: He had two half-brothers, Ptolemy Keraunos and Meleager, both of whom became kings of Macedonia (in 281 BCE and 279 BCE respectively). Both died in the Gallic invasion of 280--279 BCE (see Brennus).