[name_m]Hi[/name_m] there.
[name_m]Cedric[/name_m] is cool!
Here are my thoughts on your list for a little sister:
Rose-So lovely & classic with beautiful imagery. Ceddy & [name_f]Rosie[/name_f] are adorable together.
Florence-I love [name_f]Flora[/name_f] & Ceddy & [name_f]Flora[/name_f] are great. I find [name_f]Florence[/name_f] a bit stuffy, & prefer [name_f]Flora[/name_f] on its own. [name_f]Flo[/name_f] is not my favorite either; not sure how hazardous it is.
Camille-I like [name_f]Millie[/name_f] a lot, but really don’t care for [name_f]Cami[/name_f]. Something about it feels like a snooty girl to me. I don’t love the sound of [name_f]Camille[/name_f] though much, but it’s okay.
Adele-Hm. [name_f]Adele[/name_f] nn [name_f]Della[/name_f] almost doesn’t feel natural to me. The nn [name_f]Addie[/name_f] would feel more intuitive, but [name_f]Addie[/name_f] is so over done. [name_f]Adele[/name_f] is okay. I think it sounds kind of abrupt.
Delphine-Ha, I would think [name_f]Delphine[/name_f] Donkey would be a plus! I’ve never seen the show, but people seem to like it. [name_f]Delphine[/name_f] is not my favorite, but it’s different.
Juliette-Aw, [name_f]Juliette[/name_f] is so pretty. What about [name_f]Lettie[/name_f]? Maybe too rhyme-y with Ceddy?
Elise-Hm. I love Ellie; not that into [name_f]Elle[/name_f]. [name_f]Elise[/name_f] is okay, but kind of like [name_f]Adele[/name_f] to me. It’s too abrupt for me, but a perfectly nice name.
Vicotoire-I’m not familiar with this name. [name_f]Love[/name_f] [name_f]Vivi[/name_f] though.
Clotilde-I’m not really sure how to say this, but it’s kind of too clunky for me.
Alma-I know an [name_f]Alma[/name_f] nn [name_f]Alli[/name_f]! I don’t love Alli(e), but Ceddy & [name_f]Allie[/name_f] do feel kind of fun together.
Alice-Love [name_f]Alice[/name_f]. So pretty. Could also do [name_f]Allie[/name_f] here if you like that. [name_m]Or[/name_m] [name_f]Lissy[/name_f]?
Cleo-I like [name_f]Cleo[/name_f]. [name_m]Super[/name_m] cute & spunky. Since [name_f]Cleo[/name_f] doesn’t have any nns, what do you think of [name_f]Clementine[/name_f] nn [name_f]Cleo[/name_f]?
Margaux-Hm. I do like this with [name_m]Cedric[/name_m], but it’s not my favorite. I have an issue with too much ‘clunk’ 
I think that it does have a natural nn of [name_f]Margie[/name_f] though. I like the idea of [name_f]Margaret[/name_f] nn [name_f]Margot[/name_f] too.
Clara-Love [name_f]Clara[/name_f]. This might be my favorite from your list in general, even if it’s not my favorite with [name_m]Cederic[/name_m]. It is quite nick-name-less though.
Sorry for any repeats, but here are a few more ideas that you may like…
[name_f]Adela[/name_f] nn [name_f]Addie[/name_f]
[name_f]Amelie[/name_f] with no nns, but so pretty
[name_f]Annika[/name_f] nn [name_f]Annie[/name_f]
[name_f]Clementine[/name_f] nn [name_f]Cleo[/name_f] or [name_f]Minnie[/name_f]!
[name_f]Camellia[/name_f] nn [name_f]Millie[/name_f] or [name_f]Callie[/name_f]
[name_f]Eloise[/name_f] nn [name_f]Ellie[/name_f] or [name_f]Lulu[/name_f]
[name_f]Estelle[/name_f] nn [name_f]Essie[/name_f] or [name_f]Ellie[/name_f]
[name_f]Frances[/name_f] nn [name_f]Franny[/name_f] or [name_f]Francie[/name_f]
[name_f]Gwendolen[/name_f] nn [name_f]Winnie[/name_f]! [name_m]Or[/name_m] [name_f]Gigi[/name_f]
[name_f]Greta[/name_f] nn [name_f]Gerty[/name_f] 
also could do [name_f]Gigi[/name_f]
[name_f]Imogen[/name_f] nn Imee/Idgie/Ginny/Minnie
[name_f]Josephine[/name_f] nn [name_f]Posy[/name_f]
[name_f]Lucille[/name_f] nn [name_f]Lulu[/name_f] or [name_f]Lucy[/name_f]
[name_f]Louisa[/name_f] nn Lulu/Lula/Lola
[name_f]Olive[/name_f] nn [name_f]Livy[/name_f]
[name_f]Penelope[/name_f] nn Penny/Poppy/Polly/Nellie
[name_f]Tabitha[/name_f] nn [name_f]Tabby[/name_f]
[name_f]Vivian[/name_f] nn [name_f]Vivie[/name_f]
[name_f]Hope[/name_f] this helps. 