I have no insight or answer to your question, so sorry there.
And off topic, sorry, but the one thing I noticed is how Madara looks like the Hagoromo's brother while he's the jin of the Ten Tails. I don't know if it means anything, just an observation I had.
I don't know & that part of the story really caught me off guard.
If the sage was never said to have a brother & all of a sudden kishi says that he does indeed have a brother then he must have some sort of relevance, otherwise why introduce him at all?
this is why i don't understand introducing a brother, hope they expand upon it. the brother seems to have kaguya's hair and resembles madara (tho madara's staff is like the so6p's). and his staff looks as though it has a bat-like structure at the end. maybe he was more demon-like and malicious than hagoromo. maybe he had to kill his brother, or the juubi corrupted and devoured him like it attempted to do obito. < look at me speculating already.
I don't really get it all myself it would appear even Madara were to capture all the tailed beasts the Jubi would still be incomplete. The whole Sage has a brother thing really caught me off guard as well.