Aeris reappeared then disappeared!?

[Squall of SeeD]

I have never seen Aeris's ghost... If I go in the church after she dies I can see it?

Yes, though you can also see it if you check there after she's kidnapped by Tseng.

About the ghost while she still lives, I have seen it too. You can only see it if you visit the church at the very last moment you can before leaving Midgard.

Thanks for mentioning having seen it. I knew I couldn't have been the only one on this site that tried it.