The next generation

OK, explanation.

This is non-canon fan art, as it is set many years after the current timeline of the show.

i.e. That is not Twilight, the two fillies are her kids, Dawn and Cimmeria.

My thought here was to portray Celestia and Luna reunited and tied together by the actions of twilight, and as a thank you and a noble gesture, they each take on one of her children, two sisters as their protegees to teach and guide them throughout their lives.

And in doing so, Celestia and Luna are able to make up for what they lost when Celestia was forced to banish Luna to the moon.

Celestia, the light, Luna the dark, and Twilight ties them together.