Complex sentence: After Peter's death and Alexandar's coronation, hostilities between the brothers rose and Prince George was arrested in 1925, proclaimed insane and locked in an asylum near the city of Niš.
Split into simpler sentences: After Peter's death and Alexandar's subsequent coronation, hostilities between the brothers arose, which lead to Prince George's arrest in 1925. He was then proclaimed to be insane and locked in an asylum near the city of Niš.