Siblings are [name_u]Fox[/name_u] [name_m]Edmund[/name_m], [name_f]Marigold[/name_f] [name_f]Beatrix[/name_f], and [name_f]Sylvie[/name_f] [name_f]Winifred[/name_f]. The sib set does matter to us but it’s not the end all be all. 
They’re ALL beautiful, but I LOVE [name_f]Cordelia[/name_f] [name_f]Lavender[/name_f]! And I think she’d fit in perfectly with her siblings 
Cordelia [name_f]Lavender[/name_f] is gorgeous! 
Fits perfectly with her siblings names!
[name_f]Cordelia[/name_f] or [name_f]Petra[/name_f] both have a nature feel like your other kids. For middle names [name_f]Rosalind[/name_f] & [name_f]Pearl[/name_f] seems to fit the style of your other kids middles best, kind of classic vintage. The only two names I don’t like from your combos are [name_f]Georgiana[/name_f] and [name_f]Lavender[/name_f]. I really like [name_f]Cordelia[/name_f] [name_f]Pearl[/name_f] (Daughter of the sea goes so well with Pearl) and [name_f]Petra[/name_f] [name_f]Rosalind[/name_f] (A beautiful rose that can flourish in rocky places)
I love [name_f]Cordelia[/name_f] best for your sibset!
I really love all of these names but I think [name_f]Cordelia[/name_f] [name_f]Lavender[/name_f] works best here. 
I like [name_f]Cordelia[/name_f] [name_f]Lavender[/name_f]! It’s so lovely.
You also have 1, 2, 3 & then 4 syllable first names if you choose [name_f]Georgiana[/name_f] or [name_f]Cordelia[/name_f]. [name_m]Just[/name_m] a funny pattern I noticed.
Two syllables, sounds like WALL-thumb
Fox, [name_f]Marigold[/name_f], [name_f]Sylvie[/name_f], and [name_f]Cordelia[/name_f]
[name_u]Fox[/name_u], [name_f]Marigold[/name_f], [name_f]Sylvie[/name_f], and [name_f]Pearl[/name_f]
[name_u]Fox[/name_u], [name_f]Marigold[/name_f], [name_f]Sylvie[/name_f], and [name_f]Petra[/name_f]
While [name_f]Cordelia[/name_f] is my favorite name, I like the sound of [name_f]Pearl[/name_f] with your kids’ names.
That said, they all sound lovely together.
Cordelia [name_f]Lavender[/name_f] is perfect to complete your set but I love [name_f]Georgiana[/name_f] [name_f]Pearl[/name_f] too!
[name_f]Georgiana[/name_f] [name_f]Pearl[/name_f] is my favorite! I love seeing people talk about [name_f]Georgiana[/name_f], it’s so gorgeous. I think it sounds perfect with your other kids! I love the connection between the meanings of Georgiana (farmer) and Sylvie (from the forest)
Petra [name_f]Rosalind[/name_f] is my favorite, on its own and in the sibset!
Cordelia [name_f]Lavender[/name_f] is lovely and fits well! (I also appreciate the gentle reminder of Anne of [name_u]Green[/name_u] Gables)
Cordelia [name_f]Lavender[/name_f]!
[name_f]Cordelia[/name_f] [name_f]Lavender[/name_f] gets my vote! 
Petra [name_f]Rosalind[/name_f] is absolutely stunning! By far my favorite, both on its own and in your sibset.
But all three of these are such lovely choices, you can’t go wrong. [name_m]Can[/name_m]’t wait to find out what you go with! 
Cordelia [name_f]Lavender[/name_f] is perfection! 
Cordelia [name_f]Lavender[/name_f] is stunning!!
Cordelia [name_f]Lavender[/name_f] is absolutely gorgeous!
I adore [name_f]Petra[/name_f] — so spunky and spirited! — so my vote has to go to [name_f]Petra[/name_f] [name_f]Rosalind[/name_f].