Definitely keep
Sibyl as a name. Unless you're planning to have at least two more kids without the S sound then I believe you should stick with
Sibyl. Personally I prefer the spelling
Sybil, but since its such an uncommon name, I doubt people will confuse it with the other spelling much.
Sibyl ClementineSibyl FionaSibyl LouisaSibyl CarysSibyl MaeveSibyl JosephineSibyl GenevieveSibyl PenelopeSibyl MatildaSibyl AdelaideSibyl VivienneSibyl GeorginaSibyl Anneliese
Hope this helps!