Complex sentence: Her paternal grandparents were Jean de Montfort and Anne de Laval, and her maternal grandparents were John VI, Duke of Brittany and Jeanne of France, daughter of King Charles VI of France and Isabeau of Bavaria.
Split into simpler sentences: Her paternal grandparents were Jean de Montfort and Anne de Laval. Her maternal grandparents were John VI, Duke of Brittany and Jeanne of France, daughter of King Charles VI of France and Isabeau of Bavaria.