Just to confirm that you are not asking to recover any of your brother's data?
The following procedure will erase everything on the computer and perform a clean install of macOS. The computer will appear like new when it was first powered on.
Here is an Apple article on how to properly prepare a computer for sale (or another user):
https://support.apple.com/en-us/HT201065
You will need to boot the laptop into Recovery Mode using Command + R, or Internet Recovery Mode using Command + Option + R. You will need to erase the whole physical drive using Disk Utility. Within Disk Utility you may need to click on "View" and select "Show All Devices" before the physical drive appears in the left pane of Disk Utility.
If you are installing macOS 10.13+ (High Sierra, Mojave, or Catalina), then erase the physical drive as GUID partition and APFS (top option).
If you are installing macOS 10.12 Sierra or earlier, then erase the physical drive as GUID partition and MacOS Extended (Journaled).
Once the whole physical drive is erased quit Disk Utility and select "Reinstall macOS".