Complex sentence: His maternal grandparents were Patrick Dunbar, 7th Earl of Dunbar and Marjory de Comyn, daughter of Alexander Comyn, 2nd Earl of Buchan.
Split into simpler sentences: His maternal grandparents were Patrick Dunbar, 7th Earl of Dunbar and Marjory de Comyn. Marjory was a daughter of Alexander Comyn, 2nd Earl of Buchan.