so how are we going to theorize these 2 brothers seemingly set up for one to die and one to live? i feel like the Roots wanted to raise one ninja strong enough to carry out their agenda and these 2 brothers were the top 2 candidates. but each would not reach their respective potential without going through a survival of the fittest type training as sai has literally illustrated. each brother would face an enemy, defeat that enemy, and take on that enemy's skills. each would accumulate enough skill until they reached each other and knowingly fight to kill each other. the winner was the one who was most emotionless and most skillful...the exact traits the Roots would want out of their key ninja. that is how sai can say his brother is dead without batting an eye. it's very uchiha like except they were both pawns to become strong by an outside force while itachi was the force to play sasuke as a pawn for sasuke to reach his ultimate potential. sorry if this is all fanfic but i'm compelled to make something up cuz i dont know how long til we get flashback telling sai's whole story.