Hypothetically what would you name a sister for siblings named [name_m]Solomon[/name_m], [name_m]Jude[/name_m] and [name_f]Penelope[/name_f] who goes by [name_f]Polly[/name_f]?
Girls names we both agree on:
[name_f]Gwendolyn[/name_f]
[name_f]Maeve[/name_f]
[name_f]Girl[/name_f] names that are on the maybe list:
[name_f]Daphne[/name_f]
[name_f]Fiona[/name_f]
[name_f]Cecilia[/name_f]
[name_f]Martha[/name_f] nicknamed [name_f]Mattie[/name_f]
I think I like [name_f]Gwendolyn[/name_f] best from your list.
[name_f]Adelyn[/name_f]
[name_f]Gwen[/name_f]
[name_f]Lilian[/name_f]
[name_f]Marian[/name_f]
[name_f]Rosalyn[/name_f]
[name_f]Vivian[/name_f]
Gwendolyn fits like a glove!! Mattie is soooo cute too 
Other ideas:
Minnie
Esther
Florence
Margaret
Sadie
Abigail
Matilda
[name_f]Gwendolyn[/name_f] or [name_f]Cecilia[/name_f]
I like [name_f]Maeve[/name_f], [name_f]Daphne[/name_f] and [name_f]Cecilia[/name_f] of your options 
If I was naming a sister, I’d consider:
[name_m]Solomon[/name_m], [name_m]Jude[/name_m], [name_f]Polly[/name_f], and…
[name_f]Mavis[/name_f]
[name_f]Antonia[/name_f] (Annie, [name_f]Anna[/name_f], [name_f]Nia[/name_f], Nina)
[name_f]Eliza[/name_f]
[name_f]Edith[/name_f]
[name_f]Tabitha[/name_f]
[name_f]Flora[/name_f]
[name_f]Constance[/name_f] (Connie, Tansy)
[name_f]Viola[/name_f]
[name_f]Leonie[/name_f]
[name_f]Elspeth[/name_f]
[name_f]Anya[/name_f]
[name_m]Solomon[/name_m], [name_m]Jude[/name_m], [name_f]Penelope[/name_f] and [name_f]Maeve[/name_f] are so lovely together 
[name_f]Maeve[/name_f] is definitely my favourite from your list and [name_f]Polly[/name_f] and [name_f]Maeve[/name_f] would make a sweet pair of sisters 
[name_m]Solomon[/name_m], [name_m]Jude[/name_m], [name_f]Penelope[/name_f] and [name_f]Maeve[/name_f]
[name_f]Gwendolyn[/name_f] nn [name_f]Wendy[/name_f] would be cool. Also love [name_f]Maeve[/name_f] and [name_f]Fiona[/name_f].
I’d have a list of:
[name_f]Raya[/name_f]
[name_f]Rhoda[/name_f]
[name_f]Yvette[/name_f]
[name_f]Ramona[/name_f]
[name_f]Vivian[/name_f]
Of your list, I think [name_f]Gwendolyn[/name_f], [name_f]Daphne[/name_f], [name_f]Martha[/name_f] or [name_f]Maeve[/name_f] fit wonderfully!
[name_f]Rosalind[/name_f] (nn [name_f]Rosie[/name_f] / Sally)
[name_f]Clementine[/name_f] (nn Clemma)
[name_f]Esther[/name_f]
[name_f]Mabel[/name_f]
[name_f]Dorothea[/name_f] (nn [name_f]Dora[/name_f] / [name_f]Dory[/name_f] / Teddy)
[name_f]Josephine[/name_f] (nn [name_f]Josie[/name_f] / Sosie)
[name_f]Beatrice[/name_f] (nn Bea)
[name_f]Tabitha[/name_f]
[name_f]Louisa[/name_f]
I like [name_f]Gwendolyn[/name_f], [name_f]Daphne[/name_f], and [name_f]Fiona[/name_f].
I’m enjoying [name_f]Martha[/name_f] and [name_f]Cecilia[/name_f] from your list.
I get very different vibes from [name_f]Penelope[/name_f] and [name_f]Polly[/name_f] though which complicates my thinking.
If I was choosing I might go with:
[name_f]Ada[/name_f]
[name_f]Dorothea[/name_f] / [name_f]Thea[/name_f]
[name_f]Helen[/name_f] / [name_f]Nell[/name_f]
[name_f]Leonora[/name_f] / [name_f]Lola[/name_f]
[name_f]Millicent[/name_f] / [name_f]Missy[/name_f]
[name_f]Delilah[/name_f] / [name_f]Lilah[/name_f]
I love the name [name_f]Gwendolyn[/name_f] (Gwen for short) and it fits so well with [name_m]Solomon[/name_m], [name_m]Jude[/name_m], and [name_f]Polly[/name_f].
A few other ideas:
[name_f]Zinnia[/name_f]
[name_f]Violet[/name_f]
[name_f]Marion[/name_f]
[name_f]Gabrielle[/name_f]
[name_f]Annabel[/name_f]
[name_f]Adeline[/name_f]