[Theory] Lord Jashin is Sage's brother

[Skull Knight]

I think Lord Jashin is sage's brother.
Firstly because he can give Immortality to anyone.
Secondly he is also a god who still haven't got any panel time in Naruto.
I think Kishi is saving him for a post war arc.

 

[Shalev]

........maybe, just maybe; not doubting it, yes. =D

 

[Ψ Veritas Ψ]

already mentioned, its a possibility indeed