Complex sentence: He was the son of Franciszek Poniatowski and Helena Niewiarowska, though rumors regarding his parentage claimed that he was the son of Jan Kazimierz Sapieha, Hetman Jam Kazimierz Sapieha by an unknown Jewish woman, later adopted by Franciszek Poniatowski.
Split into simpler sentences: He was the son of Franciszek Poniatowski and Helena Niewiarowska. Some contemporary gossip claimed that he was a natural son of Great Lithuanian Hetman Jan Kazimierz Sapieha by an unknown Jewish maiden, adopted by Franciszek Poniatowski.