Congratulations
Your list is beautiful however Sylvie is my favourite she has the same majestic quality of Lumina & Magnolia. My second favourite is Willow as I think she has a similar bohemian flair. I like the one syllable middle name theme so would keep that.
Sylvie Pearl
Sylvie Quinn
Sylvie Ruth
Sylvie Blythe
Sylvie June
Sylvie Noor
Sylvie Wren
Willow Maeve
Willow Nyx
Willow Jade
Willow Eve
Willow Scout
Willow Lux
Willow Maude
You’ve had some lovely suggestions so count any repeats as me being in agreement with the berries
Lumina, Magnolia and…
Saffron ~ this was my immediate thought when I saw your sibling set. I think Saffron would be gorgeous! Saffron Claire could work. Saffron Peace could be the ultimate boho hippy dippy name that feels ever so fun
Cassia ~ has strong nature feels and has a whimsical flair. She also shares the ‘a’ ending of Lumina & Magnolia which is fun. I like the idea of Cassia Bliss.
Cassiopeia ~ similar vibe to Cassia but far more princessy and elaborate. I think Cassiopeia feels perfect very girly with your girls names. I would go with Banks or Blaise as the middle name.
Iolanthe ~ I thought Iolanthe would be so cool in your sibling set. Iolanthe is majestic, visually interesting, unexpected and has colour nature associations with the meaning ‘Violet flower’. Iolanthe Dawn would be sweet.
Clementine ~ I’m a sucker for Clementine she’s gorgeous! Clementine is bright, elegant, has a southern flair and feels very fashionable. I think Clementine would be a sweet addition. Clementine Dot would be adorable.
Sapphira ~ I thought this name could be cool addition to your sibling set she has a moody gothic vibe fitting in with the whole dark academia vibe and has the nature connections through her meaning ‘sapphire’. I like that she ends in the letter ‘a’ like your girls. She does have negative biblical connotations but people use Delilah commonly so I wouldn’t overthink that. Sapphira Dove could be cute.
Violetta ~ I couldn’t not suggest Violetta for your sibling set I think she just sounds so sweet, upbeat and gorgeously feminine. I love that she has strong nature connections, ends in the letter ‘a’ and feels flamboyant like Lumina & Magnolia but not overly so. I like the pronunciation vee-oh-let-uh however vye-oh-let-uh is also pretty! I think this one just works. I like Violetta Fleur for the ultimate flower power combination or alternatively Violetta Lark.
Finally my last suggestion is Winika! I think Winika would be a lovely addition she looks visually interesting feels similar visually too Lumina with their strong melodic sounds, distinctive cultural backgrounds but also in keeping with Magnolia through her nature associations and vintage nickname potential. I love Winika! Maybe Winika Elm, Winika Sage or Winika Snow (really embrace winter) could work.
Hope this helps!
As you can tell I loved answering this question such fun names to work with