Also, as to Logiedon's idea, it could just be that Ninfia is an all female evolution - as female eevees are rare, so it isn't super easy to obtain.
That's true. A female-only Eevee evolution is possible, but that opens the door for a male-only as well which now I'm considering less likely from both of your responses. Instead of a fabric/ribbon/pure-type they can include a specific hold-item available to evolve the Eevee; most likely a ribbon or bow-like item.
Seeing how Ninfia can be somewhat special than most Pokemon (because of its mini movie appearance) that Nintendo can put some special things implemented in the game for Eevee to turn into a Ninfia. Sort of like Shaymin into Sky-Shaymin.