She is her aunt, and both of them realize it. But when Midnight was *a lot* younger, she referred to Celestia as her 'Gammy', and it delighted the Princess so much she kept encouraging it.
In an OOC reason, it's sort of a reference as how Celestia acts as Twilight's 'mother' figure, ergo, any child Twilight would have, she would act as her 'Grandmother' figure.
(Blame Inkwell for that. He came up with it.)
And thanks, everypony, for the compliments. I never really expected to extend this, this far when I did the first one, it started as just a small thought in my brain.
I'm trying to figure out the conclusion, but the problem is, I have three options and I'm not sure which one I actually want to select.