I'm not buying it just yet. I've seen enough authors pull this kind of thing only for it to still be a red herring sort of deal somehow or other (which would probably be even worse than this sort of cheap and dirty fix to the main problem the MC is facing). But yes, it does seem like one of the logical ways, if not a satisfying way, forward for this story and, I mean cheating on her with her own friend? Even if the sister wasn't in love with Kaoru, she'd probably take her side in this mess considering they're childhood friends too.