Oh god if this doesn't end with them together I will riot. If they're not related, it shouldn't matter two shits if they grew up together and were labelled "siblings". They're open season as far as I'm concerned.
@MrMcNasty kid probably lied, as many do, that they were attacked unprovoked. And if they have an entourage with them willing to back it up not much you can do to argue your point. Take the personal victory and pretend you're sorry. That's what I used to do when shit like that happened as a kid.
@UnknownSaint111 Kids can develop crushes in elementary school. It wasn't uncommon back in the 90s when I went and it definitely can't be any less common now. The fact that they see each other that way in the first place now means "seeing each other as siblings" is merely a label with no substance to them. I mean sure, they're precious family to each other, but they never developed that desensitization to each other sexually. They've never truly seen each other as just siblings. The whole westermarck effect isn't some sort of god-given law upon us. It generally precludes us from being attracted to those whom we grow up in close proximity, be it family or childhood friends, but there are many outliers.
The funny thing about the whole westermarck effect is even though we end up desensitized to the attractiveness of our own family, we often find partners who resemble our family both superficially and in personality.