As their own distinctive crescents show, Seier de Seton and his brother Walter sprang from a second son of the house of Boulogne. Known in their Flemish homeland as Seier and Walter de Lens, they were sons of Count Eustace I ’s second son, Count Lambert de Lens, whose daughter by a second marriage (to the sister of William the Conqueror) was the Countess Judith, mother of Scotland’s Queen Maud.

 Seier’s eldest son, Walter de Lens, or Walter the Fleming as he is described in Domesday, had his chief English home at Wahull (now called Odell) in Bedfordshire. On the Firth of Forth, as heir there of his father, Seier, he was called Dougall or "the dark stranger", a nickname which was also given to his own son Walter, and duly recorded by the family’s chronicler, Sir Richard Maitland of Lethington, in 1554.

The son of the 1^st Seier de Seton, of de Lens, is known as Dougall de Setoun and his Christian name was Walter, however he was usually described by a familiar appellation in the language of the Scots people around him.  He married Janet, daughter of Robert de Quincy (and not of Roger, who lived a century later) and had a son also called Seier (2^nd) who is often confused with his grandfather.  Dougall de Setoun, was baron of the town and lands of Setoun, and he flourished in the reign of Alexander I., A.D. 1107-1124. His wife’s father, Robert de Quincy, had married Maud de St. Liz, daughter of Simon de St. Liz, Earl of Huntington and Northhampton and Maud, or Matilda, the elder of the two daughters of Waltheof, Earl of Northumberland and Judith de Lens, the niece to King William I of England.  To appreciate these ties, note that William married Matilda of Flanders, a cousin of Dougall. Likewise, Dougall’s grandfather married as his 2^nd wife Adele, or Adelaide, of Normandy, William's sister; it made the Setoun’s cousins of the King and his sons, William II and Henry I. Henry I married the daughter of Malcolm III, King of the Scots; and 2^nd, Adela of Bas-Lorraine, a cousin of Count Lambert Lens, Dougall’s grandfather.  Malcom III’s youngest son, later King David I, married Matilda the 2^nd daughter of Waltheof, Earl of Northumberland.  By all counts the early family of de Seton was well connected with many of the Royal Houses.