mhmm, I understand. I also can see this point for this. But this wouldnt be a good debate if I didnt have a (moderate) counter:
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Read were they talk about him lying about Tobi being Madara. This makes it so everything from before can be completely deminished. And if you want to go DEEPER into it, the word "brother" can also refer to someone who is someone who is of close relation, your actual brother, OR someone figuratively who is a brother figure. Ever heard of "Anonymous"? they arnt one person thought they fall under the same name. They are a mass, the rules still apply. So this could still work in a Kagami Theory.
Urm you have yet to seen tobi's face, fact is as i said, tobi could very well me Some sort of Madara zetsu hybrid..... Nothing is diminished..... he has kept a constant story , even when konan was dieing .... Kisame and itachi both recognize him as uchiha madara, and he has shown to have no problem taking his mask off, he can't to sasuke because of the transcription amaterasu.... Kishi put in the databooks Tobi = Madara why would he put that to later change the databooks, when the databooks are supposed to be fact?, The fox recognized the mask's mans eye 16 years ago during the attack.. .everything so far adds up to him being Madara,
and a brother can be taking in context of close relation... but Madara was talking about izuna,and kishi showed flashbacks of izuna