The "General Games Chat" forum, which includes Retro Game Reviews, has been archived and is now read-only. You cannot post here or create a new thread or review on this forum.
Sin's overdrive leads to an immediate game over regardless of whether you send out an aeon
If your characters are too weak to kill him before he uses it, you may need to go somewhere else and get your stats up.
I personally used Holy with Yuna and Rikku to do roughly 30000 damage each turn and used Tidus to cast Hastega and Cheer. If you still have trouble, you may want to do some of the sidequests such as celestial weapons as this will make the battles much easier.