I can easily see how one evil force could call a non-evil force evil. This is a common tactic in totalitarian regimes. By convincing your people that the enemy is evil--even if you are evil and they are not, you have de-humanized them in your peoples' eyes, and justified your war, your destruction, and your pillaging.
It could be that Jadis' sister was not evil at all.
Or, it could be as Ticket thinks--that Jadis' sister was evil also. It is also common for evil to war against itself. After all, power corrupts. The corrupt seek power. And two sisters seeking the same power in the name of two different evils is far from unthinkable.
Bravo, Erekose, for your History of Charn!!
I just love a good Narnian Heresy.
"If you believe what you like in the Gospel, and reject what you don't like, it is not the Gospel you believe, but yourself."--St. Augustine of Hippo