Britain's Royal Families: The Complete Genealogy, rev. ed. @ p 49, Isabella of Meulan, d of Robt de Beaumont, E of Leicester, no mother
The cited information was published by Pimlico Random House, London, England, 1989, 1996 (Ref: p 49, Isabella of Meulan, d of Robt de Beaumont, E of Leicester, no mother) The author/originator was Alison Weir.