FMC's sister drugged and attempted to rape MC out of jealousy but FMC "caught them" in the act mid way. She promptly took her sisters side, didn't believe and shit on the MC for a good while until sister confessed under pressure. That's where most of his initial resentment suppose to be stems from. This info however dropped in the very last part of the novel and after that it just ends with something like "and lets be careful about your sister next time". THE END
You know...if the stuff we're missing is of this quality, then I'm totally fine with them rushing the ending and skipping all that. WTF? Sister? Rape? Undeserved mistrust from her to him? Yeah, no thanks. Where the hell was ANY OF THAT explained in the prologue? Honestly, they made it seem like their problem was more of a communication breakdown. From the way you describe it, it seems the author doesn't even try to tackle THAT problem with any kind of nuance at all anyway, and if it's something that gets dropped from out of nowhere in the story, you might as well drop it completely OUT of the story.