Story number seven of the Twilight is Magic series. See author notes in first chapter for link to read order.

Short clop fic that occurs between ch 8 and 9 of TiM: Price of Victory.
Twilight and Rainbow have been married for forty-plus years. Twilight also serves as Gaia's Avatar.
Ataxia is the daughter of Spike and Rarity, also the Avatar to the god Chaos.
Shimmering Night is the daughter of Twilight and Rainbow, also the daughter of Gaia, god of life and magic.
If you read the other stories, this make sense, otherwise all I can say is just go with it and enjoy the clop.
Thanks to Electrostatic and Crystal_bombshell for help with edits!