White racism is the ultimate form of human evil within western liberalism so as such most of the replies are obviously on the son's side.

It's reddit. It was bought by zionist war mongering Jew oligarchs after grass roots users built the popular subs and made the site popular, and then turned into anti white pro Jewish propaganda that censors alternative viewpoints and bans users with opinions they don't like in order to manufacture consent and astroturf peer presser. It's filled with shills, feds and trolls, which is why you love the site so much and link to it in almost every post. Most people are conformist and will conform to whatever they believe the popular view is and offers the path of least resistance, hence the Jewish obsession with controlling and heavily censoring social media.

Without traveling to that cesspit of a website to read the already predictable comments or the likely made up story, I would do what the parent did in this situation and punish the kid for attempting to bully his sister and damage her property.

Or say if your child used a racial slur or racially assaulted a non White peer, how would you react to it and/or punish them?

If they said a racial slur I would treat it the same way as if they were swearing, although it would depend somewhat on what the supposed "slur" actually was. For the second part, what does "racially assault" mean exactly? If they physically assaulted someone and it wasnt a situation where they were standing up for themselves and others or defending themselves, I would punish them, beyond that it would depend on what exactly they did.