Siderea

Attacks in English are done more frequently with intonation than with the words themselves because you get in less trouble that way. Sadly intonation doesn't come through so well on the internet (which might be part of why attacks on the internet are "meaner", i.e. more words-based). This post mentions a case of using intonation in opposition to words (she's referring to this ad) and includes a way of writing out intonation. If the punchline was written in that manner I think it would be: your BROTHER would KNOW.

One of the trademarks of a dysfunctional family is that they tend to put each other down, and living in the family has a tendency to resemble a 'survival of the fittest' sort of scenario. Parents will often pit children against each other / compare children to each other , very rarely in a positive way.

" Your brother would know. " is sort of short for " your *brother* would know, because unlike *you* he isn't a stupid freak, and why the hell can't you do anything right and Jimmy always knows the answer so why don't you? "

... mostly. It's an attack against one child, implying they aren't as smart/good/knowledgeable as the brother.