Received used MacBook Pro & cannot install OS X

If the MBP is from Late 2011 or later, then do the following which your BIL should have done. The computer must have access to an Internet connection, preferably, Ethernet. If all you have is Wi-Fi, then be sure to sign in to your Wi-Fi account at the beginning of the process.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs, only if installing Mojave or Catalina ) or Mac OS Extended, (Journaled.)
8. Provide a volume name, usually "Macintosh HD."
9. Click on the Apply button, then click on the Done button when it activates.
10. Quit Disk Utility and return to the Utility Menu.
11. Select Install OS X and click on the Continue button.

This will install the original factory-installed version of macOS that you can later associate with your Apple ID and password as well as upgrade.