Well Caramoor, despite the fact that they are fanatically dedicated to the cause, ANYONE can be corrupted. No matter how pure the soul, and devoted to an ideal. I wont even dare say that corrupting the Sisters would be easy, but im sure it can be done. All it takes it a bit of ambition, and any Chaos God can work from that. Im sure in that entire Order, there are a few individuals who could be fooled into serving Chaos. And once you start, chances are there isnt any going back.
Anyone familiar with Diablo from Blizzard and the story surrounding the "Hero"? Thats a perfectly good example. The best intentions can lead to corruption and undoing. All that faith and all that confidence in the Sisterhood, the Order, and the Emperor can easily be used against them, just like anywhere else.
I look forward to seeing the day where some Slaanesh Worshipping Sisters rise up and start smashing down the pitiful Imperium of Man.
And again for the record...no matter who it is....be it Space Marines (we know about them dont we? *hint hint* Horus...) Eldar, or Sisters of Battle. Anyone can be corrupted. Faith is never absolute. And even if it was, it could be contorted to serve another purpose. If you think youre following your ideals, its not corruption is it? At least, not in the corrupted's eyes............... ..........