theyre not brother madara told the truth he only lied about the kyuubimaybe itachi and madara are related lol, afterall madara did say earlier on that itachi's origins began before konoha was founded. i also think madara is not who he says he is i think its his younger brother, and itachi may have been in the War at the tym of the senju and uchiha clash.
so he cudnt kill itachi bcoz they r brotherz ^_^