In its two references MCS does not say anything about Raoul except his marriage to Alice d'Eu, and their child Maud who m. Humphrey de Bohun.
Raoul I de Lusignan, d. 1 May 1219, Count d'Eu, brother of Hugh IX & son of Hugh died young (son of Hugh VIII, Sire de Lusignan & wife Bourgogne). [Ancestral Roots]
The above citation by AR indicates that Raoul is brother of Hugh IX, but not son of Hugh VIII, but the son of a recently discovered "Hugh died young" (but not too young to have fathered two sons, I name him Hugh VIII-1/2). CP and the well-respected German work "Europaeische Stammtafeln" (ES) which I don't have a copy of named him son of Hugh VIII. CP, which is a little outdated on this particular area would still have him as brother of Hugh IX, while ES has him as uncle of Hugh IX, with Hugh IX being son of Hugh VIII-1/2. Following the postings on the newsgroup soc.genealogy.medieval, does not lead to a definite conclusion as to what the "experts" know about Raoul's parentage.
Given the fact that the experts are divided as to Raoul's parents, I feel somewhat justified in following the majority of the World Connect people in having Raoul's parents being Geoffrey & Humberge. I have no published source for this, indeed the sources say he was son of Hugh VIII or Hugh VIII-1/2, with many experts arguing that he was not a younger brother of Hugh IX because then Hugh IX would have taken the very lucrative marriage with Alice d'Eu instead of Raoul. Placing Raoul as son of Geoffrey makes him 1st cousin to Hugh IX.