Originally Posted by Phat Chocobo

I think Jenova's main goal all along was to merge with the Lifestream energy and corrupt the entire Gaiasystem, but she was prevented from doing so either because the Cetra stopped her, or because she was just physically impossible to do because she wasn't originally from that world, and therefore "incompatable" with the Lifestream.

But acting through Sephiroth and twisting his motives to do what she couldn't - she would take control after all, while all the time Sephiroth would be thinking that it's his own will that wants him to merge with the Lifestream and become a God, not realising that Jenova was influencing his judgement.

That way the Jenova that was a part of Sephiroth would fulfil it's main goal, and the rest of Jenova would follow, through the reunion.