[name_m]Oscar[/name_m] [name_m]Leopold[/name_m] and [name_f]Cordelia[/name_f] [name_f]Iris[/name_f] are such a gorgeous sibset! They feel literary, vintage and a little whimsical, with lots of character. I think you have a really lovely list already!
My standouts are [name_f]Daphne[/name_f] [name_f]Allegra[/name_f], [name_f]Ariadne[/name_f] [name_f]Juliet[/name_f], [name_f]Beatrix[/name_f] [name_f]Ivy[/name_f] and [name_f]Olive[/name_f] [name_f]Matilda[/name_f]. [name_f]Daphne[/name_f] [name_f]Allegra[/name_f] feels particularly beautiful to me, elegant but not too serious, and I love the rhythm of it. [name_f]Ariadne[/name_f] [name_f]Juliet[/name_f] is distinctive and sophisticated, while [name_f]Beatrix[/name_f] [name_f]Ivy[/name_f] has a bit more spunk and charm. [name_f]Olive[/name_f] [name_f]Matilda[/name_f] is so sweet with [name_m]Oscar[/name_m] and [name_f]Cordelia[/name_f], too!
I also really like [name_f]Dahlia[/name_f] [name_f]Penelope[/name_f] and [name_f]Athena[/name_f] [name_f]Rosemary[/name_f]. [name_f]Dahlia[/name_f] has that lovely botanical feel without being too trendy, and [name_f]Athena[/name_f] [name_f]Rosemary[/name_f] has a wonderful balance of strength and softness.
I’m intrigued by [name_f]Winter[/name_f] as a middle name! I actually think it could work beautifully with some of your choices. [name_f]Daphne[/name_f] [name_f]Winter[/name_f], [name_f]Beatrix[/name_f] [name_f]Winter[/name_f], [name_f]Olive[/name_f] [name_f]Winter[/name_f] and [name_f]Matilda[/name_f] [name_f]Winter[/name_f] are all lovely. [name_f]Daphne[/name_f] [name_f]Winter[/name_f] is probably my favourite of those, it feels a little magical alongside [name_m]Oscar[/name_m] [name_m]Leopold[/name_m] and [name_f]Cordelia[/name_f] [name_f]Iris[/name_f].
A few other names I’d consider for your list are [name_f]Theodora[/name_f], [name_f]Isadora[/name_f], [name_f]Florence[/name_f], [name_f]Felicity[/name_f], [name_f]Helena[/name_f], [name_f]Imogen[/name_f], [name_f]Sylvie[/name_f], [name_f]Magnolia[/name_f], [name_f]Ottilie[/name_f] and [name_f]Rosalind[/name_f] . I particularly love [name_f]Theodora[/name_f] and [name_f]Isadora[/name_f] with your older two. For combinations, perhaps [name_f]Theodora[/name_f] [name_f]June[/name_f], [name_f]Isadora[/name_f] [name_f]Pearl[/name_f], [name_f]Florence[/name_f] [name_f]Beatrix[/name_f], [name_f]Helena[/name_f] [name_f]Winter[/name_f] or [name_f]Magnolia[/name_f] [name_f]June[/name_f]?
One thing I’d keep in mind is the overall balance of the sibset. [name_m]Oscar[/name_m] is familiar and punchy, [name_f]Cordelia[/name_f] is elaborate and romantic, and I think a name like [name_f]Daphne[/name_f], [name_f]Beatrix[/name_f] or [name_f]Olive[/name_f] could sit beautifully between those styles. [name_m]You[/name_m] certainly don’t need to match the others too closely, though!
If I had to narrow your current list down to three, I’d choose [name_f]Daphne[/name_f] [name_f]Allegra[/name_f], [name_f]Beatrix[/name_f] [name_f]Ivy[/name_f] and [name_f]Ariadne[/name_f] [name_f]Juliet[/name_f] . They each have a slightly different feel, but all sound wonderful with [name_m]Oscar[/name_m] [name_m]Leopold[/name_m] and [name_f]Cordelia[/name_f] [name_f]Iris[/name_f]. 
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, Olive Francesca)
