Congratulations! How lovely to see you around the forums again 
[name_f][/name_f] Your children’s names are gorgeous.
For their sister, I particularly like names that are different in sound but share the same quirky, retro storybook vibe. For me, that rules out popular [name_f]Emilia[/name_f] and trendy Latinate [name_f]Aurelia[/name_f]. [name_m]Reading[/name_m] the above replies, I can see [name_f]Christabel[/name_f] is a popular choice and I do find it classy and original, but it’s perhaps a tiny[name_f][/name_f] bit fussy with your others.
[name_f][/name_f][name_f]My[/name_f] top 5 for you (in no particular order):
[name_f][/name_f][name_f]Mabel[/name_f][name_f][/name_f] [name_f][/name_f]— [name_f]Sweet[/name_f], streamlined, old-fashioned but not outdated. Quite trendy here in the UK but still under the radar in the US.
[name_f][/name_f][name_f]Wilhelmina[/name_f][name_f][/name_f] [name_f][/name_f]— [name_m]Even[/name_m] longer than [name_f]Christabel[/name_f] but somehow feels less fussy, due to the sleek sound and strong meaning. [name_f]Lovely[/name_f] nicknames [name_f]Willa[/name_f], [name_f]Mina[/name_f] and maybe even [name_f]Billie[/name_f].
[name_f][/name_f][name_f]Matilda[/name_f][name_f][/name_f] [name_f][/name_f]— Definitely storybook, definitely strong, has that slightly quirky flair and is a [name_f][/name_f] pan-European choice that feels particularly fitting with S@bina and Brun0.
[name_f][/name_f][name_f]Margot[/name_f][name_f][/name_f] [name_f][/name_f]— A little bit trendy at the moment but nowhere near as popular as [name_f]Emilia[/name_f] and with no soundalikes, which make it feel chic and distinctive.
[name_f][/name_f][name_f]Annabel[/name_f][name_f][/name_f] [name_f][/name_f]— Close call between [name_f]Annabel[/name_f] and [name_f]Philippa[/name_f] for my final spot. Both have that sweet but substantial, slightly quirky [name_f]Brit[/name_f] feel. But ultimately I like that [name_f]Annabel[/name_f] has a different ending sound and sparky nickname [name_f]Annie[/name_f] would be lovely with your gang.
[name_f][/name_f]Good luck!