I have a longer form take on this but:
1: Idea seeded through pornography to soften up innate resistance and disgust response
2: Many children grow up with maybe a single sibling, 2 at most. Good chance they have no opposite sex sibling to learn from, allowing the fantasy to exist.
3: Difficult dating landscape devoid of deep seated intimacy and longstanding love makes them daydream about someone loving them truly & permanently.
4: They see themselves as gross/unlovable/broken so the fantasy of a pretty person being there, forced by familial proximity to love and protect is attractive.
(The protection aspect is HUGE w/ girl incest-fans, they are scared of normie boys and their sexual aggression whether real or imagined. Girls w/ brothers and good fathers rarely have the fujoshi-incest fetish IME)
RT: https://goreslut.xyz/objects/84e9443a-b4d6-4a36-8c26-f679b79ba711