This feels... lost, like the author just doesn't really know what to do with the sister anymore and is just going from idea to idea. At first she's cunning, manipulative, and cruel, secretly collecting evidence of Kaho's(Well, I guess they're both Kaho but we all know what I mean) films and presenting it to their mother to get her in trouble, seemingly with no regard for her well being. Then she's someone who ignores being cummed on while she sleeps, suggesting a history of sexual abuse, and we get hints about a dark backstory as well. Then she's a dumb, but protective older sister who's whole plan was to reveal Shirasawa's evil nature to protect her younger sister from him. Then she's shocked and delighted when Shirasawa finds her attractive, being NOT used to sexual things, and falls hard for him because she's overwhelmed by the attention. THEN she's jealous of her younger sister, and now she's bold enough to come back to his house, wants to see the movies he's making, and recognizes the smell of semen and the very idea that Shirasawa masturbated into those tissues is enough to send her into overdrive.
That's not even getting in to how Kaho straight up made out with her older sister and didn't seem to care about it. It feels like he has no real idea of the sister's role and is kind of just rolling with what gets good audience reaction, or is fun to write and draw.
Which... is a real shame. There's a strange, sweet story in here between the two leads, two people figuring out their passion, testing each other's boundaries, and realizing their attraction to each other. There's a story about a son trying to figure out what the legacy of his father actually means, both to him and others. There's something here and it's been lost for a while now, struggling to surface beneath a chapter-to-chapter definition of the older sister's character as the author struggles to figure out just who this girl is and how we should feel about her.