I created a new user for FTP on my Cube, then changed "ftp_user" home folder from /Users/ftp_user to /Users/chrisv/Public/Drop Box in Net Info Manager.
That way, I can access all files dropped there by people logging in as "ftp_user" without having to log in as a different user to get to them.
Turn on ftp sharing in the sharing pref pane, and if you're using OS X's firewall, make sure you allow ftp in the Firewall tab. Your brother should be able to log in at
FTP://your.ip.address and username and password would be whatever you named and assigned your ftp user that you set up.
Just don't make your ftp user an admin, or give it permission to launch apps and it should be fairly secure. They won't be able to access any of the other folders in your users folder, and for added security, you could set access to the System and Applications folders to "others: no access" if you're the only user of your computer.
CV