An iPhone is designed to be synched to exactly one computer. You chose to use your brother's computer, so now the iPhone will act as if it's his.
There will be an issue because you want his music and other data, but they belong to him. if that were not the case, I would suggest doing Settings > General > Reset > Erase All Content and Settings on your iPhone and starting over with your own blank slate (using your own iTunes to set up your iPhone).
Maybe you should start by reading the "basics" area of the iPhone User Manual:
iPhone User Guide (For iOS 6.1 Software)