Whenever you start warband through the steam library it switches the registry to the CD key that belongs to the steam account youre oN. if you want to play on your brothers computer, get on your steam account on that computer and launch warband and it should auto change the cd key to yours. If you DONT want to and youre on your brothers cd key, start up warband through his library and then when youre on the warband launcher go into the registry and paste ur cd key in the serial_key (Which is located in the registry from HKEY_CURRENT_USER ---> Software---> Mount&BladeWarbandKeys ---> serial_key) then start up the game. If you want to use the cRPG launcher which doesnt use steam, you can start it like normal and change ur cd key whenever u start up the game (IF you have the wrong cd key on)