My favorite with your subset is [name_u]Sky[/name_u], as it is short, sweet, and unique, just like [name_f]Fay[/name_f] and [name_f]Leia[/name_f]. I think [name_u]Sage[/name_u] and [name_f]Lily[/name_f] are a bit too popular with your sibset, and that [name_f]Arielle[/name_f] and [name_f]Elowen[/name_f] are a bit too long. I like [name_f]Luna[/name_f], but not as much as [name_u]Sky[/name_u], and I think that more tailored [name_u]Ariel[/name_u] might be a better fit with [name_f]Fay[/name_f] and [name_f]Leia[/name_f] than [name_f]Arielle[/name_f]. However, I think [name_u]Sky[/name_u] [name_f]Arielle[/name_f] or [name_u]Sky[/name_u] [name_f]Elowen[/name_f] would make great names! Other ideas:
[name_f]Gaia[/name_f]: This name originated a very, very long time ago, and is unique and substantial like [name_f]Fay[/name_f], but is super bright and spunky like [name_f]Leia[/name_f]. On the surface, it may seem a bit too close to [name_f]Leia[/name_f], but not if both names are pronounced the most common way: [name_f]Leia[/name_f] is typically said “[name_m]Lay[/name_m]-uh,” while [name_f]Gaia[/name_f] is pronounced “[name_m]Guy[/name_m]-uh,” so the middle vowel is different. Plus, [name_f]Fay[/name_f] and [name_f]Leia[/name_f] already share a long “a” sound, so this could be a good way to tie it together. [name_f]Fay[/name_f], [name_f]Leia[/name_f], and [name_f]Gaia[/name_f] are very cool sisters!
[name_f]Juno[/name_f]: This name most certainly qualifies as mythological, and packs lots of style and substance into few syllables, just like [name_f]Fay[/name_f] and [name_f]Leia[/name_f]. [name_f]Fay[/name_f], [name_f]Leia[/name_f], and [name_f]Juno[/name_f] are distinctive, beautiful sisters.
[name_f]Fern[/name_f] or [name_f]Briar[/name_f]: These names are both vintage nature names that are sweet, but also strong. I like how they subtly pick up the sounds of [name_f]Fay[/name_f] and [name_f]Leia[/name_f].
[name_u]Wren[/name_u] or [name_u]Rue[/name_u]: I think these lively-yet-gentle nature names find a balance between the unique vintage qualities of [name_f]Fay[/name_f] and the playfulness and relative popularity of [name_f]Leia[/name_f].
[name_f]Nova[/name_f]: A brief yet stylish name, and one of the brightest out there. Nature ties due to the stars.
[name_f]Hope[/name_f] this helped!