This can all be explained with the "Chaos Thoery."
...
...wait, I don't know the Chaos Thoery...
...
Okay, so it can't be explained...


Anyway, Tidus, Jecht, Tidus's mom, etc... all existed when Zanarkand got destroyed, and the Fayth just happen to remember them. I guess it could be possible that Shuyin is somehow a decendant of them. That's how I see it anyway, until proven otherwise.