i gave the thought of danzo being Madara's brother pretty serious thinking and came to the conclusion this is most likely not true, but i will leave the possibility open,
iv rested on the idea the Danzou was the student of madara before he fought the 1st.
Danzou and the 3rd are piers and the 3rd was trained by the 1st so it leave's the possibility that madara could of been the teacher of danzo, it can help explain the deep hatred he has for the 1,2,3,5 th hokage, other then the fact he is aginst the "will of the fire" mentality.
iono i could see it being written into the story line in the next chapter or two,
it could also explain why he has a efing shargian arm, he saw the power first hand of the most powerful uchai, madara, and wants that power
just an idea