If I understand the circumstances right, Cindy was bitten the exact same time as Peter was, but was shut away from the world from that moment until recently. Emotionally, she's still that 15/16 year old, with no real world experience with love, attraction and lust beyond the high school level---so she could very well think of Peter as an "ex". Peter, being way more experienced, is gonna recognize that inexperience and be protective of her. He might even like her as a person, but I'd think he's way too smart to get close to her and confuse the issue.