This is going in a story but it's such a long time down the road I'll answer it anyway! And thank you very much for your interest. Alec realized he wanted Isabelle when she was about five or six (ambiguous because the feelings were ambiguous at first) which was kind of alarming for him because before that point he'd never felt desire for anyone younger than about eleven. Alec was very disconcerted by how she was an exception for him, and the degree of "power" she had over him, in his own view. Isabelle meanwhile didn't have a clue and wouldn't catch on for years.