This question contains unmarked spoilers for season six of My Little Pony: Friendship is Magic. By now, most people should have seen the episodes here discussed, but just in case avoid the question if you fear spoilers.
Before season six come in, most people assumed that there were two kinds of alicorns:
- born alicorns like Luna and Celestia
- turned alicorns, is ponies that weren't born as alicorns but become alicorns due to some deed they performed (Twily).
In the season six premiere, we meet a new "born alicorn" - Flurry Heart, Cadence daughter. In that scene, Celestia says the following words:
"The birth of an alicorn is something Equestria has never seen"
Those word were taken as an indication that not even Celestia and Luna were born as alicorn but instead turned into alicorn afterwards. This would make Flurry Heart the first "natural" alicorn in the series.
But... is this really the case? Does the aforementioned quote really imply that Celestia and Luna weren't alicorn at birth? As far as I get it, this only implies that during the existence of Equestria, no new alicorns were born. When Equestria was first founded, its flag already featured the two sisters (see season 2 ep 11 - Hearth's Warming Eve) so even by disregarding the Journal of the Two Sisters (which canon status isn't really agreed upon by everyone) we have an in-show reference to alicorns existence before Equestria. Celestia words could simply mean that since Equestria was created, no new alicorn was born, indicating that she and her sister were the ast two of their kind.
Am I forgetting something in my analysis? Or does this really mean that the common conclusion many made after that episode is flawed? Are there any additional source on the matter? Did season six really "rewrite" the two sister origin and implied they too were regular ponies at some point?