MacBook autofill name fields with someone else's name
This likely pertains to the Address Book card marked as "me". Launch the Address Book application (from Spotlight, Finder, Launchpad, or the Dock), create or locate your own card, and choose the Card > Make This My Card menu item.

This could've originally happened if you or someone else created an Address Book card for your brother and marked it as "me", or if you copied Address Book data from another computer belonging to your brother.