Complex sentence: The good Polybius, King of Parthia, is the protector of both his own daughter Lisinga and her lover Siveno, who is believed to be the son of Minteus, a minister of King Demetrius of Syria, but is actually the long estranged son of Demetrius.
Split into simpler sentences: The good Polybius, King of Parthia, is the protector of both his own daughter Lisinga and her lover Siveno. Everyone believes Siveno to be the son of Minteus, a minister of King Demetrius of Syria, but he is actually the long estranged son of Demetrius.