Complex sentence: He overthrew his older half-brother, King Stefan Radoslav with help from his father-in-law, the Bulgarian emperor (tsar) Ivan Asen II, and assumed the Serbian throne after being crowned by his uncle, Serbia's Archbishop Saint Sava.
Split into simpler sentences: He overthrew his older half-brother, King Stefan Radoslav with help from his father-in-law Ivan Asen II of Bulgaria. He assumed the Serbian throne after being crowned by his uncle, Serbia's Archbishop Saint Sava.