|The simple solution is disable the wireless network off when you want to game. This can easily be done from the router's management interface. It change be re-enabled when you're done in a matter of a minute or two.|
Other than that, check and see if he's doing any file sharing at all when online on his laptop. If he is, that will chew up bandwidth like nothing else will. If he's not then it's the combination of the video (another bandwidth hog) and the wireless network.
Wireless REALLY sucks by the way. You pay for the convnenence with an inherent lack of security and susceptibility to lag and latency. If you're connecting wirelessly to game you'll want to change that to a wired connection. If you ARE gaming over your wireless network, changing to wired will probably fix all lag issues you're having even while your brother is online with his laptop.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.