William was the younger son of Lord Murdoch Alexander, Earl of White Haven. He had an older brother, Hamish, and was the brother-in-law of Emily Alexander and Honor Harrington, as well as the paternal uncle of their children, Raoul and Katherine Alexander-Harrington. (HH11) Theodore Harper, Grand Duke of Manticore, was one of his cousins. (HH10) He was married, and he and his wife had several children. (infodump)