The Castle of the Two Sisters existed before Equestria was founded.

[Shady Mist 55]

According to Ocellus/Twilight, The Castle of the Two Sisters plus Celestia and Luna existed before Equestria was founded. Unless it had a different name, sense this would be before Starswirl became their teacher and they would be how young and "maybe" live with their parents. 3:10-3:16 The keyword being "Pre-Equestrian".

 

Edited October 2, 2022 by Shady Mist

[Toastypk 920]

This was shown in the comic, too!

The Pillars had a comic series that showed the old castle when Celestia and Luna were younger.

[Megas 27,064]

The continuity in general is such a mess lol

[diamondtiger71 3]

In "The Journal of the Two Sisters" it states that Celestia and Luna were asked by Starswirl to be the princesses of the newly-formed Equestria due to their alicorn-hood. After they accept, Celestia and Luna decide to move from where they grew up in Canterlot and build the Castle of the Two Sisters in the Everfree Forest. (Before Ponyville was founded nearby.)

Officially, the books are considered canon unless contradicted by the show. The Castle of the Two Sisters being mentioned in Twilight's history of pre-equestrian friendships is pretty odd, but perhaps she was just mentioning one of the earliest attempts to unite multiple tribes? (AKA Starswirl's attempt to better unite all 3 pony tribes via alicorn rulers.)

[Courageous Thunder Dash 7,809]

This had to take place just months after Discord was defeated. But then now that begs the question...where would the Hearths Warming story fit in? 

[Shady Mist 55]

13 hours ago, Courageous Thunder Dash said:

This had to take place just months after Discord was defeated. But then now that begs the question...where would the Hearths Warming story fit in? 

Discord's rule had to have been after Equestia was founded and the defeat of the Pony of Shadows sense the Pillars were formed and trapped in limbo before The Tree of Harmony and Elements were fully grown and used by Celestia and Luna to defeat Discord.