MacBook not letting me log out of Guest Account
If you're not positive that your bro has Guest Account access set up, OS X introduced a new feature related to 'Find My Mac' that creates a sandboxed fake Guest Account in order to make it easier to recover your mac in case it gets stolen, luring the thief to go and connect to some random Wi-Fi with the guest account.

Maybe that's what's going on, you could easily restart and login to any account you like though.