Alexander was the first cousin of Walter the Steward, father of King Robert II of Scotland, and of Sir James Douglas, two of the most important commanders during the First War of Scottish Independence. He was also the brother-in-law of Thomas Randolph, 1st Earl of Moray, nephew of King Robert I of Scotland. His mother was Margaret de Bonkyl, the daughter of Sir Alexander de Bonkyl of that Ilk. Alexander's wife was Jean Fitz James [Jean's surname should be "fitz Alexander" as the custom was "fitz" (daughter of) or "Fitz" (son of) then the father's Christian name in this case "Alexander" hence "Jean fitz Alexander"], daughter of Alexander Fitz James, with whom he had two children, a son and heir, John Stewart, and a daughter, Isabella Stewart, who married Domhnall II, Earl of Mar.