Celestia's relationship with her dear sister is that of a double edged sword. One small push in either direction will cut the small thread that still holds them together. However, it is not so bad that such a action should take place any time in the foreseeable future. That is, before it showed up

Now It's a race against the clock, as Luna struggles to keep above the sea of uncertainty, as the anchor planted by the spirit that haunts her slowly begins to tug her down below the icy waves of her psyche.