it can also be cause by your isp. some like virgin media prioritize traffic.
iplayer and the likes get high priority. as do voip and online gaming.
p2p and normal surfing get a lower priority... so it may be down to what you were doing at the time you took the measurements.
also your brothers wirless may be telling the router it has a higher priority than other wireless devices.
if he is on a desktop and is using a dedicated card this will be easy to test.
just remove it and install it into your pc. and see if you get the same or higher bandwidth.
Alright, i'll try it. Thanks for the info.