Plus, I understand how people find it weird especially if y'all are westerners especially white ppl.
But... Calling someone older than you even strangers 'brother', 'sister', 'uncle', 'auntie', 'grandpa', and 'grandma' are pretty normal in Asia. The context here is obviously different, since she never hold that as a big brother title and see him as brother. It's just a title to her

I don't know why people here keep commenting how messed up or fu*ked up the story is not realising it already is FROM THE COVER OF THE MANGA.
Also, I do understand some of y'all read due to hype or good art, or how hot the characters is. But common sense need to be applied when you're approaching stories like horror and psychology. There's a reason for why there's no shojou/romance/josei inside here.
This was never a romance nor cutesy shit for readers. Additionally, I don't know why y'all have been so disgusted by incest shits going on when things like this are common in Japanese manga or any fiction guilt trip stories.
also, IT'S HAS BEEN STATED WAYYYY LONGGGG IN EARLY CHAPTER THAT HE NEVER SEEN HER AS SISTER. and she also sees him as a character and never a brother. It's pretty obvious from the start how it's narrated FROM THE FIRST FUCKING CHAPTER.