Alec always knew that if he had children he would probably sexually abuse them, simply because he knows himself and how much of a selfish, ruthless bastard he is, not to mention that he and Bertrand already broke the incest taboo when they were children. So he sort of assumed from the start with Isabelle, long before he was attracted to her. This plus his obsession with her starting when she was five led to years of grooming and eventual sexual abuse from age twelve upwards. He raped her for the first time when she was fourteen.