my brother linked his Xbox to my Bungie but he doesn't have the Xbox anymore it broke and no I can't get it off for cross-save because he doesn't remember his info.
Please have your brother contact Microsoft concerning recovering the account. That'd be the best course of action for you.
Sorry but if he can not remember the security codes then your options are limited. If you still had a Xbox then you could talk to Microsoft to help you to regain access to your account but now your only real option is to create a new Bungie.net account and link the accounts you do want to it. Please note you will lose any history your old account has such as any emblem codes you have put in.