Complex sentence: Both George III and the Duke of Kent died in January 1820, making the Prince Regent George IV and Drina third in line to the throne after her uncles, the Duke of York and Duke of Clarence (the future William IV).
Split into simpler sentences: Both George III and the Duke of Kent died in January 1820. The Prince Regent became George IV and Drina was third in line to the throne after her uncles, the Duke of York and Duke of Clarence (the future William IV).