Just so you know most of my edits are fixing typo's , putting in words I missed or rewording things to sound better/ make more sense
While I went the route I did as Celestia would have being a ruler to put simple would need to almost constantly act like things are fine and nothings wrong.. while keep doing her duties.. so without her sister around to confide anything in and the like she had to have pent up matters growing .. thus when their parents showed up it would have logically been the breaking point for her and any façade she had going
So this more or less how I envision Luna finaly turning into Nightmare moon happened
https://www.youtube.com/watch?v=tao1Ic8qVkM
Also this song is largely how I think Celestia would have felt after banishing Luna
https://www.youtube.com/watch?v=pQIhyGYUDmI
Also heres a fanon that fell into decline years ago and I think its one of the view fanons in any fandom that had vanished or atleast almost has as I can't seem to find a trace of it outside of one very rarely popping up and this one is the following Celestia and Luna basically has a Phoenix like ability to die and be reborn again from the ashes in Celestias case or moon dust in Lunas case .. Thus being the key to their span on the world .. Granted I am not planning to use that fanon but if you want to or think its a nice touch we could