Linksys WRT54G models have free firmware updates that add QoS/Reserved Bandwidth that should let you sort of accomplish this. Check www.linksys.com
for the firmware update for your unit and instructions.
This will work if you want to limit the bandwidth by port traffic. For example, Web, FTP, Kazaa, etc. You can go into the "QoS" section and reserve bandwidth for your web surfing. If your brother is downloading stuff via other means (Usenet/Newsgroups, FTP, P2P, etc.) then what I have described will work just fine.