I LOVE [name_f]Hosanna[/name_f] “Honey”. How sweet and hopeful. I think [name_f]Sylvie[/name_f] is my other favourite from your list because she matches so nicely with [name_f]Willa[/name_f], [name_f]Elowyn[/name_f], and [name_f]Florence[/name_f]. I agree with @persephonescrown that a subtle nature theme would be nice.
But honestly all of them are nice. I think [name_f]Fern[/name_f] feels a bit too close to [name_f]Florence[/name_f] imo as I often think of [name_f]Fern[/name_f] as a [name_f]Florence[/name_f] nickname, but a lot of berries may disagree.
My suggestions:
[name_f]Leonie[/name_f] - a bit feistier than her sisters, but just as pretty
[name_f]Oona[/name_f] / [name_f]Una[/name_f] - a softer choice, but I love the meaning lamb. I think [name_f]Oona[/name_f] and [name_f]Leonie[/name_f] would be a great choice for subtly [name_m]Christian[/name_m] twins, for the lion and lamb imagery.
[name_f]Coralie[/name_f] - More sea than forest, but I think it’s nice with [name_f]Elowyn[/name_f] especially.
[name_f]Primrose[/name_f] “Posie” - I really like Posy/Posie for your set, but I think a longer name like [name_f]Primrose[/name_f] would be needed.