This story is a sequel to The Origin Of Moonbutt
Twilight Sparkle is now aware of Princess Celestia's nickname being 'Sunbutt,' and Princess Luna's nickname being 'Moonbutt.'
But the answer to one question still eludes her: Does Cadence have a nickname?
With this in mind, she decides that maybe it's time she paid her dear sister-in-law a visit...
(Takes place before Season 6. Reading the prequels is not required.)