Wait how is she letting people die in a plate fall?
It circles back to my theory that Aerith wanted the same as Sephiroth; to defeat the Fate (can I just call them this?) so that she has a chance to live. It's a gamble now, but she has a chance. Problem is, Sephy has a chance, too. If she was aware of it and there was a chance to... change/kill/whatever the fuck else, why not do it sooner and give a chance for the Platefall to be stopped?
(yeah, yeah, I know, devs wanted the Midgar portion to be similar and then change stuff, blah, blah... I'm just pointing out how her words of "change Fate" in the late chapters and her bizarre behaviour at times throughout the game point to her knowing stuff and especially that she was going to die -her speech in the dream sequence yell "I'm dead/I know I'll die" with a megaphone in a microphone- and she was shocked at Corneo's words. So, either she didn't know about the platefall exactly, but knew about Sephiroth and her death because Ancients, or she knew and didn't prioritize it.
Again, yeah, yeah, I know, devs wanted the finale of the game to be "Fate dies" or sth to have an excuse to change shit in next part and dear God this is so stupid and wow, unfortunate implications rise why does this Fate thing happened Christ)
Sephiroth's not in the Lifestream the same way he is in Advent Children. Sephiroth in FFVII is alive. He essentially came back to life by reforming his physical body thanks to Jenova cells, and is in the Lifestream, using its power to continue to grow stronger.
In Advent Children, he was a spirit. He had no physical form and was essentially a vengeful ghost who laid a planetary curse upon humanity. Via disease.
Defeating Sephiroth while Aerith is still alive (I assume that this is what she wants) still makes Geostigma happen while it also prevents it from being healed. So no, I don't think that she has thought that far. Or, has seen that far. Her face bookened the OG, after all. What I'm saying is that she doesn't want the OG to play as it did (how meta), because it's a bad thing for her. She's very aware of a lot of stuff.