WaxerRed I know this is an old one, but I strongly second the "it cannot be Hester" idea. I can add more evidence of it NOT being Hester (and Alanon's sister does seem like a reasonable suspect!):
1) Hester is against the use of the Church as a political force, exactly the opposite of what the cold, dogmatic light would almost certainly want. How do we know? Because she argued against it when it would count - both during the Ardford summit (she wants to revisit what she sees as the overly high influence of the Inquisition on the nations of Arclent) and later during the succession crisis, when she makes a speech against the Church having a role in politics (hurting Jhenno's chances).
2) She actively argues -against- overly dogmatic positions taken by her fellow High Priestesses. In fact, when we first meet her, she is strongly arguing against Andra's support of the crackdown on prostitution. She doesn't need to do this to maintain cover, she could simply have left it alone, so it seems to be a look into her personality. She is pragmatic, not overly doctrionaire.
3) Hester is not above working even with incubus kings, like the Doom King, if she feels she is fighting a greater evil. She commits the church to cooperating with Simon against both the Incubus King/Emperor and the Fucklord. She seems to have no particular hatred of succubi or orcs either.
4) She has a very revealing personal chat with Carina and basically tells her not to worry about what Ivala would think, but to do whatever she, Carina, believes is actually best for the world - even if that means working for an Incubus King. This strikes me as being literally the opposite of what the shadowy figure controlling Ivala herself would want.
In summary - we've seen enough of Hester to have an idea of her personality. Based on what we've seen, even in circumstances where Hester would have no reason to "pretend" to be more reasonable, the cold sterile light of "Ivala" is something that would actually strike Hester as a threat.
In fact, I doubt that this hidden person would be a High Priestess at all - that position is too visible. Better to leverage one's divine power to essentially force the Church leaders to act according to your own wishes....