You’re square on the money, in fact. Only Aura and Nicaea existed until after the main campaign was done. A playtester suggested adding a third just to complete the reference. At the time, the chapter 22 red gem was on a generic wyvern lord that people tended to miss. Thus, I thought I’d kill two birds with one stone and signpost the red gem with a third sister miniboss.
So yeah, if she feels awfully redundant, that’s because she is. She exists to complete the pegasus trio reference and to make the red gem more visible, and that’s it.
As a side note, I initially created Aura with the intent to have her be the playable pegasus knight. Theorycrafting with her quickly hit a wall, though, as generic pegasus knights are one of my least favorite FE archetypes and I couldn’t come up with anything interesting for her. Then I came up with the Nicaea concept, and the rest is history.