Q: How do I completely erase all content and settings

You can't. Your brother will need to recover his password. There is help for that: If you forgot your Apple ID password - Apple Support

The following has instructions what to do then: What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

He needs to recover the password for the Apple ID so Find My iPad can be turned off and you can erase all content and settings. Have him go to https://iforgot.apple.com