Two big issues here:
1) Your account.
Typically an Apple ID is an email address you have used. If you are currently using a mobile device that is logged into your account, go to Settings and tap your name at the top.
Look in Keychain, and in a browser's security settings if you use it to automatically sign into Apple when using a browser.
This support article describes ways to find your Apple ID --> If you forgot your Apple Account primary email address or phone number - Apple Support
or go directly to this link --> https://iforgot.apple.com/appleid
If you find you can’t sign in, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple Account password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity. More detail in —> Activation Lockout Issues - Apple Community
If you still need help:
Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."
Contact Apple Support - Apple Support
2) Do not use your brothers iPhone! The content of your two accounts could be permanently intermixed. If you do need to use it, sign in through a web browser. By signing into his device, I hope he knows that he essentially made a gift of it to you.
