Complex sentence: He was son to James Stewart, 5th High Steward of Scotland by his spouse Cecilia, daughter of Patrick Dunbar, 7th Earl of Dunbar and Marjory, daughter of Alexander Comyn, 2nd Earl of Buchan.
Split into simpler sentences: He was son to James Stewart, 5th High Steward of Scotland and Cecilia Dunbar. His maternal grandparents were Patrick Dunbar, 7th Earl of Dunbar and Marjory de Comyn, daughter of Alexander Comyn, 2nd Earl of Buchan.