If your brother can't remember the password, you'll have to erase the phone and restore either from a previous backup or as a new device. Apple has a support page for just this situation. http://support.apple.com/kb/ht1212
If your brother had find my iPhone turned on, he'll have to enter his Apple ID and password to erase the iPhone.
If you have synced your device with iTunes, you can restore your device.
1. Connect the device to the computer you normally sync with.
2. Open iTunes. If iTunes asks you to enter the passcode, try another computer that you've synced with.
Or try the steps below to use recovery mode.
iTunes will automatically sync your device and create a backup. If it doesn't, sync the device with iTunes. After the sync is complete, restore your device.