Remove incest tag, they are not real brother and sister, they just treat eachother like that.
Also it isn't really NTR, friend boy was content being treated like a brother, he only got mad after she hooked up with a guy. It's more a story of "you snooze you lose", he thought she would eventually just start going out with him without saying anything.
Really big positive is tan boy doesn't do any of the really awful shit like passing her around to his friends or something.