Complex sentence: His paternal grandparents were Jean I of Albret and Catherine of Rohan, a daughter of Alain IX, Viscount of Rohan and Marguerite of Brittany.
Split into simpler sentences: His paternal grandparents were Jean I of Albret and Catherine de Rohan. Catherine was a daughter of Alain IX, Viscount of Rohan and Marguerite of Brittany.