Complex sentence: In a series of dynastic alliances, Maria Antonia became engaged to Ferdinand, Prince of Asturias (who later became King Ferdinand VII of Spain), while her eldest brother, Francis, became engaged to Ferdinand's sister Infanta Maria Isabella of Spain.
Split into simpler sentences: In a series of dynastic alliances, Maria Antonia became engaged to the Prince of Asturias. This same Prince later became King Ferdinand VII of Spain, while her eldest brother, Francis, became engaged to Ferdinand's sister Infanta Maria Isabella of Spain.