If you plan to use your brothers account you won't have access to his Keychain, you will want to go and make a new one otherwise it will nag you for his old password, which you don't have.
A better plan would be to:
In the Apple Menu > System Preferences > Account, use the Plus and minus to...
Immediately make an Admin Account, and then log into that.
From there make a standard account for your normal use.
Log back into your brothers account, make a backup of his files, especially iPhoto photos. You might want to get an external FireWire Drive for this.
Then log into your normal account and Delete your brothers account. This will require the Admin Password, but you know that because you made the Admin Account;-)
Work in your account and when it needs an Admin Password just enter the Admin Password you made. You should never really need to login to the Admin Account.