Good morning and thank you for posting in the Final Fantasy XI Technical Forums.

We apologize for any inconvenience experienced when trying to play Final Fantasy XI. For us to better assist you, please respond with the following information:
What Platform are you using? (Steam/Windows/etc.)
What Platform is your brother using?

At what point does the issue occur?
Have you ever been able to log onto FINAL FANTASY XI?

Are you running any additional programs alongside Final Fantasy XI?

All steps taken to resolve the issue:

If possible, would you be able to try removing the local files to wipe the login details and try again? Please navigate to 'Program Files(x86)' > 'PlayOnline' > 'SQUARE ENIX' > 'FINAL FANTASY XI' > 'USER' and copy this 'USER' folder so it may be stored elsewhere as a backup. Once complete, proceed to remove sub folders out of this main folder one by one (Not the backup) so you may determine if there may be corrupt files.

Alternatively, we may recommend removing the folder completely and performing a file check from the PlayOnline Viewer so the client may recreate/restore this folder. Please be advised that removing the 'USER' folder will also remove any macros that may have been created.

Thank you for visiting the Final Fantasy XI Technical Support Forums!