how to access internet with Linux <=> WinXP

First off, kill the USB thing. There is no advantage.
Second, you need to configure the kernel on the linux box to do network address translation (NAT) with iptables. Next, make sure you have 2 network card. connect the cable modem up to the first card (eth0), and then connect your brother's computer to you computer with a cross over cable, or connect into the switch using a regular network cable. set up iptables to do nat, and block outside connections (ie, a firewall), and set your XP ip addr to something like 192.168.0.2, and the the second interface card (eth1) on the linux box to 192.168.0.1. all of this is can be found in the linux how-to's. www.tldp.org
let me know if you need more direction.

"the most important thing is not life, but the good life" - Socrates