One method that would stop this would be to create a password for your brother's account. Afterwards, it will no longer auto sign-in to the account. This would be the recommended method.
Another method is to use step 1 in OPTION ONE of the tutorial below to always have all users displayed on the sign-in screen to prevent this. However, if you click on your brother's icon, it will still sign into it without having to enter a password since it's not password protected as recommended above.