What sibling names sound nice with [name]Mabel[/name] [name]Frances[/name]? No criteria right now, so just go for it! 
[name]Mabel[/name] [name]Frances[/name] and…
[name]Clara[/name] [name]Beatrice[/name]
[name]Georgia[/name] [name]Eleanor[/name]
[name]Eliza[/name] [name]Genevieve[/name]
[name]Stella[/name] [name]Miriam[/name]
[name]Anna[/name] [name]Cordelia[/name]
[name]Susanna[/name] [name]Jane[/name]
[name]Julia[/name] [name]Caroline[/name]
[name]Eva[/name] [name]Marguerite[/name]
[name]Ivy[/name] [name]Madeleine[/name]
[name]Annabel[/name] [name]Josephine[/name]
[name]Alice[/name] [name]Tabitha[/name]
Boy’s names, just in case:
[name]Jonathan[/name] [name]Lucas[/name]
[name]Leo[/name] [name]Alexander[/name]
[name]Elias[/name] [name]James[/name]
[name]Arlo[/name] [name]Sebastian[/name]
[name]William[/name] [name]Cyril[/name]
[name]Henry[/name] [name]March[/name]
[name]Oliver[/name] [name]Matthew[/name]
I really like [name]Mabel[/name] [name]Frances[/name], btw. 
What about:
[name]Iris[/name] [name]Virginia[/name]
[name]Josephine[/name] [name]Louise[/name]
[name]Margaret[/name] [name]Mary[/name]
[name]Olivia[/name] [name]Roxanne[/name] (might be a little too modern)
[name]Phoebe[/name] [name]Scarlett[/name]
hope you like them!
irisrose I love your suggestions and these are the pick of them.
[name]Georgia[/name] [name]Mary[/name]
[name]Cordelia[/name] [name]Hope[/name]
[name]Susannah[/name] [name]Ellen[/name]
[name]Julia[/name] [name]Caroline[/name]
[name]Alice[/name] [name]Tabitha[/name]
Boy’s name
[name]Jonathan[/name] [name]Lucas[/name]
Fun! I apologize for repeats…
[name]Alice[/name]
[name]Amelia[/name]
[name]Beatrice[/name]
[name]Cora[/name]
[name]Evelyn[/name]
[name]Flora[/name]
[name]Ivy[/name]
[name]Matilda[/name] (M theme, though)
[name]Nora[/name]
[name]Phoebe[/name]
[name]Rose[/name]
[name]Violet[/name]
[name]Willa[/name]
[name]Arthur[/name]
[name]Charles[/name]
[name]Conrad[/name]
[name]Calvin[/name]
[name]Dexter[/name]
[name]Edward[/name]
[name]Felix[/name]
[name]Henry[/name]
[name]Leo[/name]
[name]Miles[/name] (M theme, though)
[name]Oliver[/name]
[name]Phineas[/name]
[name]Peter[/name]
[name]Simon[/name]
[name]Theodore[/name]
[name]Truman[/name]
[name]Wilson[/name]
I hope you’re doing well!
[name]How[/name] about [name]Matea[/name]? It’s the feminine form of [name]Matthew[/name] and means ‘a gift from God’.
[name]Matea[/name] [name]Lilli[/name]
[name]Matea[/name] [name]April[/name]
[name]Matea[/name] [name]Shaun[/name]
[name]Matea[/name] Novelle (Latin for ‘new; a bright star’)
[name]Matea[/name] [name]Joie[/name]
What do the rest of you think of the [name]Matea[/name] anyway?
Great idea [name]Carter[/name]! I am so going to copy!!
the very first name I thought of was [name]Alice[/name]!
My great grandmother and her siblings were:
[name]Florence[/name] [name]Alice[/name] [name]Ethel[/name] “[name]Florrie[/name]”
[name]Alfred[/name] [name]Arthur[/name]
[name]William[/name] [name]John[/name]
[name]Dorothy[/name] [name]Beatrice[/name] “[name]Dolly[/name]”
[name]Gertrude[/name] [name]Maude[/name] “[name]Gertie[/name]”
[name]Ruby[/name] [name]Kate[/name]
[name]Mabel[/name] [name]Lily[/name]
So they were the first names that spring to my mind.
Also I would add:
[name]Clara[/name] [name]Lillian[/name]
[name]Eliza[/name] [name]Matilda[/name]
[name]Violet[/name] [name]Evelyn[/name]
[name]Ada[/name] [name]Genevieve[/name]
[name]Ellen[/name] [name]Amelia[/name]
[name]Beatrice[/name] [name]Helen[/name]
[name]Edith[/name], nn [name]Edie[/name]
[name]Phoebe[/name]
[name]Lucinda[/name], nn [name]Lucie[/name]
[name]Clara[/name]
[name]Nell[/name]
[name]Penelope[/name], nn [name]Nell[/name], [name]Nelly[/name], [name]Penny[/name]
[name]Hazel[/name]
[name]Elsa[/name]
[name]Eliza[/name]
[name]Charlotte[/name], nn [name]Lotte[/name]/[name]Lottie[/name]
[name]Etta[/name]
[name]Harriet[/name], nn [name]Hattie[/name] or [name]Etta[/name]
[name]Evelyn[/name], nn [name]Evey[/name]