The people who are siscons are in most cases guys who have no clue what it's like to live with a sister, be it of any age. They just think it's like another woman living in the same house as you, when in reality you are biologically hardwired to not be attracted to her. Maybe some guys have this forbidden fruit fetish where they find it hot, because it's so taboo, but that's all and the the thought of it is more attractive than the person itself. This is mostly what happens to pornsick people. As soon as you clean your head, you'll realize how insane of a thought it's to be attracted to your sister. Not because something rational, but a deeply hardwired biological thing.
inb4 but how come some people actually are attracted to their sister
well, those people are mentally ill or otherwise have gotten that hardwiring overwritten by some pathology like porn addiction or cultural nurture. no normal person is attracted to their sister.