Complex sentence: He was the son of James Stewart, 5th High Steward of Scotland by his spouse Gilles (or "Egidia") de Burgh, daughter of Walter de Burgh, 1st Earl of Ulster.
Split into simpler sentences: He was the son of James Stewart, 5th High Steward of Scotland. His spouse was Gilles (or "Egidia") de Burgh, daughter of Walter de Burgh, 1st Earl of Ulster.