Hi Sepanta,
Thank you for posting, I am here to help. To fully remove your brother account please follow this steps:
Log in using your brother account, Create another user and switch it to admin.
Click Start button and open Settings. Click account then choose manage account. Add new user set account type as admin. log out to your brother account and log in using your new account.
Remove Windows credential:
Click Start button and search for Windows credential. Remove all generic access for your brother Microsoft account. Once done go to Settings again and open Account remove your brother account.
Log in to Microsoft account and remove saved devices:
Advise your brother to log in to his Microsoft account and go to Devices page make sure to check your device name and match it to your brother Microsoft account under device name is the same on his end. Once confirmed delete / remove the device.
Restart your Computer and log in your user account created earlier. Kindly Check if this will work.
Let me know how this goes. Looking forward to helping you out. If you need more help please feel free to let me know. If the answer is helpful, please click "Accept Answer".
Best regards,
Jansher