Quote Originally Posted by BG-57
Each Summoner's Final Aeon can defeat Sin, but the Final Aeon becomes reborn as the next Sin.
It's actually something of an oversimplification to say that the Final Aeons are "reborn as Sin." The FFX Ultimania Omega guide makes it clear that Sin is comprised from a vast amount of pyreflies, which suggests to me that the erstwhile Final Aeon acts more as a "heart," an anchor for the overall pyrefly-comprised armor. Yu Yevon himself possesses the Final Aeon, which allows for the "reconstruction" of Sin. Prior to the first Final Summoning, Yu Yevon himself (retaining a fully corporeal form) would have acted as this anchor.