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According to Hagoromo, Ashura specifically chose Naruto to reincarnate due to his personality. Its like he deemed Naruto worthy enough to be his successor. Their personalities and lifes are similar which made Ashura chose Naruto.
It wouldn't be a mistake to assume that it happened the same with Sasuke and Indra as well. So when did this two brothers decided to possess them ?
It wouldn't make sense if it was when they were babies or really small.
-For Sasuke I think that was around the time their fight with Naruto at VOTE. Indra decided that 'Power' is the answer, and around that time Sasuke decided to do anything to obtain 'Power'.
-For Naruto I guess it was some time after the Mizuki incident or his fight against Gaara.
I'm at my limit of Sakura threads this day so lets not drag her here please.