I've never heard of
Cecil on a girl before. I confess I'm pretty rigid on gender-bending names. And when adorable, bouncy
Cecily is available I don't understand why parents need to use
Cecil on a girl.
But the fact that is was your great-grandmother's name is definitely an important factor for you. So I can understand you wanting to use it. I would think it would be easier for your children to live with if you used
Cecilia /
Cecily for a girl and kept
Cecil for a boy. But anything goes now so who's to say?
Just out of curiosity, do you know how your gret-grandmother pronounced it? "SEE-sil" or "SESS-il?" And was she in the U.K., Canada, U.S., other? You don't have to answer that. I'm just nosey that way.