My brother forgot to unpair my watch from his phone while moving to a different country. How can we unpair his id and add mine to the watch again???

Have him do the following;

Remove  Watch Activation Lock -

Go to icloud.com and login

Open Find iPhone.

In the All Devices pulldown menu select your Watch.

In the resulting sheet that appears to the right choose erase Apple Watch. Select Next until the device is erased.

Then Remove the watch from your Apple ID.

This should reset the watch to the factory settings for you to setup as new and pair it to your iPhone.

M1 Mac mini, macOS 11.3.1; Watch, series 5 GPS/cellular, watchOS 7.4.1; iPad Pro 2nd Gen, iPadOS 14.5.1; iPad Air 2, iPadOS 14.5.1; iPhone 12 Pro, iOS 14.5.1; TV 4th Gen, tvOS 14.5  HomePods, audioOS 14.5 and Airport Express w/ Airplay 2