(SPOILER)The first one could have been, and the makers of Vegnagun KNEW it. However, since Vegnagun was sealed away, and Yevon deceived the people, so by making Yunalesca do the Final Summon, she sealed the defeat of the people of Spira. While her intentions may have been good at one point, being dead changes your opinion on things, and she becomes a tool of her father. She is so disoriented, she becomes a fiend in FFX-2, after revealing her true colors in FFX. (Very Ultimecia type battle it was, and in FFX-2, she is on level 80 of the Via Infinito, and is now Chac, a Basilisk type monster)

The second Sin was made from an Aeon, rather than being one in its essence. Yu Yevon then learned how to reincarnate Sin using the fallen Final Aeon. So after this, Sin would have been weak, but since Machina was forbidden at that point, and everyone agreed to the letter, Sin grew in power. Yu Yevon then leveled up Sin to level 100, and put all his Skill Points in Anti-Machina defense, since the only other method was letting him use Aeons, and he had already been able to reincarnate Sin using the Aeon that defeats him.

FFX is one gigantic conspiracy (HAHAHA GET IT CONSPIRACY? LMAO!

You are now dumber for having read that. I bid you good day.