The ancestry for Raedburh is not at all sure. The following post by Chris Bennet to soc.genealogy.medieval indicates that she was related to Charlemagn in some way:
There is no contemporary source for Egbert's wife. The name "Raedburh" comes from a medieval manuscript quoted by W. G. Searle (Anglo-Saxon Bishops, Kings and Nobles, London 1899, 343) as "MS Trin Coll Oxf x". She is described there as "regis Francorum sororia" which can only be a reference to Charlemagne; however there is no such sister known. Since Egbert was certainly in exile at Charlemagne's court, it is perfectly plausible that he married his wife there. If the tradition has any value, then the best guess is that she was probably a sister-in-law, but she could just as well have been a lady of the Court. Settipani (La prehistoire des Capetiens, 308 n791) is willing to accept the name.