What do you think?

[Rowellen]

I like Cicily, Cecelia, & Celia. I kinda like the spelling Cecily a bit better though.

Others:
Celeste
Celestine
Celine/Celina/Selina
Serina/Sarina
Ceyla
Sylvia
Simone

ETA: I would probably avoid names that rhyme with older siblings names personally.

[TaRhLi.]

We know a Cecilia and she gets called Cec (pronounced Sess), so I suppose you would need to think about that name being shortened as it is long and people will shorten it especially if they have trouble around a 4 syllable name.

[Rowellen]

That reminds me, the Cecelia I know gets called Cele (pronounced Seal).