Depends on the pronunciation, surely?
I'd use
Cecily because
Cecil is a family name on both sides; plus, I prefer the e sound to the i sound here.
Cecily AnneCecily BeatriceCecily ClareCecily Dorette
Cecily EvaCecily Frances (sounds lovely, but two fem forms of male names are two too many)
Cecily GiselleCecily HelenCecily ImogenCecily JeanCecily KarenCecily MargaretCecily NorahCecily OlwenCecily PrudenceCecily RuthCecily SusanCecily ThereseCecily Valmae
Cecily Winona