Sorry ..see what you mean ..it can be spelled both ways .
In antiquity, the oracular seeresses of the Ancient Near East and the Mediterranean were referred to by the Greek term "sibyls". In modern times, when "Sibyl" is adopted for a woman's name, the conventional spelling is "Sybil".
I think they got that clue a bit wrong in that case .
I put Sybil ..no doubt it is what they want .