Looking back the root of this seems to be the parents treating Kouta like the trash and Shizuku like the perfect golden child who can do no wrong- her sympathy for Kouta is possibly what turned her feelings from normal sibling love to wanting to fuck him and him being treated like dirt by his other family members almost definitely is what made Kouta vulnerable to being groomed by her. Gonna be interesting to see if any of the truth (be it the fact that Shizuku, not Kouta, is the actual groomer/predator here or the fact that it's basically all the parents' fault) gets through to mom and dad in the next few chapters.