GOG is a little slower to update the game, since Larian can update Steam directly, but needs to send the patch to Steam for them to check and upload (they write their own installers, as well). There should be an update early next week for both versions, which should hopefully bring them up to to par.
Once that happens, you might want to set Steam to not automatically update, so you can delay any future updates until your brother has access, as well.
