Hmm, seems a difference between the two probably exists, but yeah, may not be a Mac issue. Could be hardware, software, router, network settings, conflict issue, etc.
Bandwidth may be a part of the problem, but you'd have to have pretty bad internet (56k modem quality) to get affected that heavily by just another person playing WoW. Speedtest as mentioned will be a good test.
Is your brother playing WoW the only activity which causes your latency to increase? Does he use the internet for other things? If so, how do those activities affect your gameplay? Are you both using wireless? Only one of you uses wireless? Bah, too many questions to ask
