Correct me if I’m wrong but I don’t remember anything about him with her older sister nor her ever talking about an older sister. I think you’re mistaking it with a different manga. I read one similar to the one you’re talking about and it’s not this one. The art is kind of similar so I can see why, but he’s just a big overprotective bear who really loves her.
I think you missed the whole manga.
I might be wrong but i think that you confused it with "pretending to sleep can't stop him"?
Are you perhaps thinking of this (hot piece of garbage): http://www.mangago.zone/read-manga/pretending_to_sleep_can_t_stop_him/
Your thinking of a totally different manga
Yes exactly chapter 77. God i’m so clumsy
I guess i missed a few chapters but he was together with her sister first right? Correct me if i’m wrong i still see him as scum that he cheated on her sister with her. The had to break her sister’s heart like that. (The black haired girl, i think) i still think that all cheaters should die.