Suggestions for a sister for ?
No biblical names or frilly girly names, thanks.
Nature-inspired names to compliment [name]Calla[/name]:
[name]Iris[/name]
[name]Juniper[/name]
[name]Hazel[/name]
[name]Rowan[/name]
[name]Flora[/name]
Names I think “go” with the sibset as a whole:
[name]Aidan[/name], [name]Finn[/name], [name]Calla[/name] and…[name]Leona[/name], [name]Daphne[/name], [name]Phoebe[/name], [name]Nora[/name], [name]Lillian[/name], [name]June[/name].
[name]Aidan[/name] [name]Finn[/name] [name]Calla[/name]
[name]Olive[/name]
[name]Blair[/name]
[name]Mia[/name]
[name]Lilac[/name]
[name]Holly[/name]
[name]Wren[/name]
[name]Junia[/name]/[name]Juno[/name]
[name]Aidan[/name], [name]Finn[/name], [name]Calla[/name] and…
I like pp’s suggestions of [name]Flora[/name], [name]Phoebe[/name] or [name]Nora[/name].
Or how about:
[name]Dahlia[/name]
[name]Amaryllis[/name] (too frilly?)
[name]Briar[/name]
[name]Blythe[/name]
[name]Lucy[/name] or [name]Lucia[/name]
[name]Madelyn[/name] or [name]Madeleine[/name]
Names I like for you but not sure they go with sibset:
[name]Florence[/name]
[name]Louise[/name] or [name]Louisa[/name]
[name]Eleanor[/name] 
[name]Aidan[/name], [name]Finn[/name], [name]Calla[/name] and…
[name]Heidi[/name]
[name]Willow[/name]
[name]Olive[/name]
[name]Rowan[/name]
[name]Lila[/name]
[name]Juno[/name]
[name]Ruby[/name]
[name]Isla[/name]
[name]Iris[/name]
[name]Rose[/name]
[name]Stella[/name]
[name]Shiloh[/name]
[name]Hadley[/name]
[name]Lilac[/name]
[name]Flora[/name]
[name]Wren[/name]
[name]Mila[/name]
[name]Quinn[/name]
[name]Aidan[/name], [name]Finn[/name] [name]Calla[/name] and…
[name]Tallis[/name]
[name]Britta[/name]
[name]Dinah[/name]
[name]Hazel[/name]
[name]Honor[/name]
[name]Maura[/name]/[name]Mara[/name]/[name]Mora[/name]
[name]Nora[/name]
[name]Nola[/name]
[name]Pearl[/name]
[name]Thea[/name]
I think the following would go nice with your brood:
[name]Maeve[/name]
[name]Fallon[/name]
[name]Siobhan[/name]
I also like the suggested [name]Maura[/name], [name]Daphne[/name] and [name]Nora[/name].
[name]Aidan[/name], [name]Finn[/name], [name]Calla[/name] and …
[name]Blaire[/name]
[name]Maren[/name]
[name]Aidan[/name], [name]Finn[/name], [name]Calla[/name] and…
[name]Liv[/name]
[name]Avery[/name]
[name]Jill[/name]
[name]Elle[/name]
[name]Aidan[/name], [name]Calla[/name], [name]Finn[/name], and…
[name]Laurel[/name]
[name]Athena[/name]
[name]Scarlet[/name]
[name]Melody[/name]
[name]Charlotte[/name]
[name]Maya[/name]
[name]Lily[/name]
[name]Savannah[/name]
[name]Gemma[/name]
[name]Fiona[/name]
[name]Audrey[/name]
[name]Bridget[/name]
[name]Georgia[/name]
[name]Ivory[/name]
[name]Autumn[/name]
[name]Summer[/name]
[name]Helena[/name]
[name]Ivy[/name]
[name]Noelle[/name]
[name]Stella[/name]
[name]Alice[/name]
[name]Julia[/name]
[name]Gillian[/name]
I immediately thought of [name]Delphine[/name]. Lots of lovely suggestions though!
[name]Maren[/name]
[name]Estelle[/name]
[name]Stella[/name]
[name]Lily[/name]
I don’t know if you want to pick something Irish-ish. [name]Aidan[/name] and [name]Finn[/name] fit that theme, but not [name]Calla[/name]. Personally, I would pick something else (not [name]Fiona[/name], [name]Maura[/name], [name]Bridget[/name], etc.) because you’d end up with an odd one out.