My understanding of this is that, just like any other force power, Zannah's spells would have literally no effect on Sidious (assuming he is prepared for it) due to him being completely out of Zannah's league as a force-wielder in general.
I think of how Skere Kaan tried to influence Bane's mind with telepathy but ultimately failed, with Kaan describing it as "a rusty knife trying to pierce thick leather armor" (or something along those lines). Even though Bane had never before been exposed to Kaan's abilities, his far superior standing in the force (as Kaan admitted earlier) meant his mental 'barriers' couldn't be pierced.
Eragon fans will understand this.