I wonder if his network settings changed...
If your connection automatically get settings from the router,
your brother's computer should also be the same. If you manually
set the settings, your brother's computer should also follow the
same.
Otherwise, you will get connections issues.
Start > Control Panel > Network Connections
Right click on the connection and select
Properties. Double click on Internet
Protocol (TCP/IP)
If you are getting settings automatically, check that it's the
same for your brother's. If it's not, change it to automatic.
If you are using manual settings, change the last IP digit.
Example:
If you are using 192.168.1.5, your brother should be using
192.168.1.6