Complex sentence: His brother Bolesław took over the government of all his domains; after the birth of Przemysł II, he remained under the tutelage of his uncle until 1273, when he received Poznań as his own district.
Split into simpler sentences: His brother Bolesław took over the government of all his domains. After his birth, young Przemysł II remained under the tutelage of his uncle until 1273, when he received Poznań as his own district.