She is in constant direct contact with their Goddess Dibella and through her the cult knows the mind/will of their god.
[About Sybil]
Sybil does not remember her true name,
once the prior woman selected by the Cult of Dibella had died, s
he was chosen to become the vessel
It is to no surprise that Sybil is extremely beautiful,
so much so that she had a multitude of fans,
but mostly stalkers who would do anything to have her for their own.
Living and Undead
In the middle of the night,
a group of Forsworn had captured her and intended to defile her.
However there was a stronger being who wanted her for his own.
Argos
While captive, a mysterious chill eradicated the Forsworn, and grabbed Sybil by the throat.
She was his now.
He continues to keep her for his own...
Sybil can be found within the Temple of Dibella in Markarth
Frost mage
UNP Body
HDT and XP32 skeleton required
Custom Dress and Outfit which explains the ritual of Dibella
Custom Frost spells with unique effects
Argos' Ward which shields her in combat and Argos' Grasp where he literally sinks his claws into his enemies
Finally when things seem grim, Sybil can conjure Argos a powerful mage to assist her in combat.