Britain's Royal Families: The Complete Genealogy, rev. ed. @ If she was sister to the King of Franks, she would be a sister
of Charlemagne.
The cited information was published by Pimlico Random House, London, England, 1989, 1996 (Ref: If she was sister to the King of Franks, she would be a sister
of Charlemagne.) The author/originator was Alison Weir.