Complex sentence: Suleiman's previous heir apparent, his eldest son Mustafa, had been strangled to death 13 years prior at the sultan's order.And his other son Bayezid, had been killed by his support and Selim II's order in 1561 with his 4 princes.
Split into simpler sentences: Suleiman's previous heir apparent, his eldest son Mustafa, had been strangled to death 13 years prior at the sultan's order. His other son Bayezid, had been killed by his support and Selim II's order in 1561 with his 4 princes.