When you invite someone to join WikiTree and they accept, they become the Profile Manager of their account profile. You are removed as a manager.
Your brother could add you back as a manager. He'd just need to go to his profile page, click the Privacy tab, scroll down to your name on the Trusted List and select the [add as manager] link.
As long as you are on the Trusted List, though, there's very little you can't do. You won't be able to change the Privacy Level or delete his e-mail address, but you can do just about everything else.