Ancestral Roots of Certain American Colonists Who Came to America Before 1700, page 116
Line 23. FULK IV, 'Rechin,' Count of Anjou, b. 1043, d. 14 Apr. 1109; m. (5) 1089, Bertrade (or Beatrice) de Montfort (div. abt. 1092), d. 1117, dau. of Simon I, d. abt 1087, Seigneur of Montfort L'Amauri, and Agnes of Evreux, dau. of Richard, Count of Evreux, and gr. grandau. of RICHARD I (121E-20) Duke of Normandy. (CP XI: App.d, 114)

She was the daughter of Simon I de Montfort[1] and Agnes of Evreux. Her brother was Amaury de Montfort.

In speaking of Fulk IV, Count of Anjou, the chronicler John of Marmoutier would recount:
The lecherous Fulk then fell passionately in love with the sister of Amaury de Montfort, whom no good man ever praised save for her beauty.
Bertrade and Fulk were married,[1] and they became the parents of a son, Fulk.
However, in 1092 Bertrade left her husband to live with King Philip I of France. Philip married her on 15 May 1092, despite the fact that they both had spouses living. He was so enamoured of Bertrade that he refused to leave her even when threatened with excommunication. Pope Urban II did excommunicate him in 1095, and Philip was prevented from taking part in the First Crusade.
According to Orderic Vitalis, Bertrade was anxious that one of her sons succeed Philip, and sent a letter to King Henry I of England asking him to arrest her stepson Louis. Orderic also claims she sought to kill Louis, first through the arts of sorcery and then by poison. Whatever the truth of these allegations, Louis succeeded Philip in 1108.