Complex sentence: On his return to Thebes, he married Alcmene, who gave birth to twin sons, Iphicles being the son of Amphitryon, Heracles of Zeus, who had visited her during Amphitryon's absence.
Split into simpler sentences: On his return to Thebes, he married Alcmene, who gave birth to twin sons, Iphicles and Heracles. Only the former was the son of Amphitryon because Heracles was the son of Zeus, who had visited Alcemne during Amphitryon's absence.