If the wireless network is already set up on your laptop to connect to wireless at home, then when you take it somewhere else with another wireless network it should pick it up and ask if you want to connect to it. It should then connect and ask for your brother's security key (if he's set one up, if he hasn't it's something that needs doing).
If you haven't got a wireless connection set up at home, you sometimes need to press a function key on the laptop to turn the wireless adapter on. It should then pick up your brother's network.
If your laptop hasn't got a wireless adapter, you'll need a wireless PCMCIA card for it - click here.