Some modems have the standard Ethernet connection and a USB connection. Some allow both to be used at the same time, some don't. You'll just have to investigate the modems available to you. If a modem will allow both connections, connect his using the USB port, and then connect a router using the Ethernet port for the other PCs.
I think I'd have a heart to heart with my brother.
My brother has the only computer connected to the internet in the house and refuses to let us set up a home network because he's paranoid. Is there any way to hook up both a wireless router and his computer to the cable modem seperately? I'd hate to have to get a whole seperate cable hookup.