I disagree for precisely the reason that it wouldn't be incest if sacramentalist had either abused his stepdaughter as a child or had formed a relationship with her as an adult. The former would be disgusting and wrong and paedophilic. The latter would have been creepy and also disgusting and wrong and paedophilic if his attraction to her formed when she was a child and he just "waited" till she was of age. But it wouldn't be incest.