I love [name_f]Betsy[/name_f], [name_f]Fern[/name_f], and [name_f]Rowena[/name_f].
I like your choices of [name_f]Francine[/name_f], [name_f]Rowena[/name_f], and [name_f]Fern[/name_f] with your other children 
I like [name_f]Fern[/name_f] and a short version of [name_f]Rowena[/name_f]- [name_u]Rowan[/name_u]
Hmm. I would immediately take off [name_f]Catherine[/name_f]; I think it’s too classic and stately, while your other children’s names have a whimsical side. I’d also take off [name_f]Frances[/name_f]–it’s not too classic, but [name_f]Francine[/name_f] is better. Personally, I’m not fond of nicknames-as-names, so I’d eliminate [name_f]Betsy[/name_f] as well, though I don’t necessarily think it’s a bad fit.
That leaves me with [name_f]Francine[/name_f], [name_f]Rowena[/name_f], [name_f]Fern[/name_f], and [name_f]Louisa[/name_f]. I really think it depends on what kind of vibe you’re really going for. [name_f]Louisa[/name_f] is wonderful, but it kind of leaves out [name_u]Pepper[/name_u] since [name_m]Arthur[/name_m] and [name_f]Marianne[/name_f] can also be seen as very vintage/antique names.
I think my favorite, however, is [name_f]Fern[/name_f]. It’s sprightly, whimsical, vintage, and sweet. Also, I noticed all your babies have Rs in their names, and I like that connection. [name_f]Francine[/name_f] and [name_f]Rowena[/name_f] would be tied for second.
I’d suggest [name_f]Tallulah[/name_f], [name_u]Ruby[/name_u], [name_f]Ramona[/name_f], [name_f]Lorna[/name_f], and [name_f]Blanche[/name_f]. [name_u]Ruby[/name_u] and [name_f]Fern[/name_f] would be my first picks.
My favorite is definitely [name_f]Francine[/name_f]. [name_u]Love[/name_u] it so much!
[name_f]Louisa[/name_f] is a close second. Congratulations to you!
Thanks for all the feedback! I had to take [name_f]Francine[/name_f] off the list after someone pointed the [name_m]Arthur[/name_m] children books also have a character named [name_f]Francine[/name_f].
[name_u]Baby[/name_u] will be here soon, will let you know what we choose.
I was feeling [name_f]Catherine[/name_f] is too formal with sister [name_u]Pepper[/name_u], but then saw your update of nn [name_f]Kitty[/name_f], and think this would work well. I adore [name_f]Rowena[/name_f], and also think [name_f]Frances[/name_f] and [name_f]Louisa[/name_f] are lovely and would work - especially if they have cute nns. While [name_f]Fern[/name_f] is not a favourite of mine I think it would work very well with your family.