technically it shouldnt matter which steam account you are on i think, solely the key might matter, maybe you can use your brothers warband key and he starts using yours
(you can view and change your key in regedit)
also make sure to be logged in the website and that your ingame character name matches the one on the website 1:1, capital letters, symbols, numbers etc, everything has to be an exact match for the character you wanna play
the characters gender has to be the same like when you played on your brothers steam account i think
if you did all that and the problem persists, then i guess its either another problem or actually the steam account, even though that would be weird since some people dont use steam at all to play and it works for them (thats why it should be the key instead of steam)