Several things might untangle the apparent paradox of the name.
Words change over time: an older form of this word is despite-ful, not meaning a narrow-minded bearer of malice or a person so brim-ful of bile that it starts to spill everywhere, but one who is just full of fight in a just cause. (The aircraft name "Spitfire" comes from the same root, when you think about it).
In sixteenth century Britain, "despiteful" would have meant the mood in which the country chose to fight off a Spanish invasion, despite the invaders having potentially overwhelming force; some religious believers, in this frame of mind, chose to remain Roman Catholic despite the certainty of torture and burning if they were discovered.
in this sense "spite" is the bloody-minded "sod-you" attitude that comes from knowing you are right, but also suspecting your cause is also doomed to disaster against overwhelming odds: just because they have overwhelming odds doesn't mean that they are right, nor does it oblige me to bow the knee to them. I have nothing but spite (ie, contempt) for their attitude. It's only later on that the word came to signify a malicious attitude, a curdled grievance, or a personality determined to exact a dollar's worth of retribution for a penny's worth of injury.
Perhaps the Sisters endured religious persecution in the past, withstood it, and carry the name as a proud memory of a battle won?
--AgProv 12:26, 21 May 2008 (UTC)
Considering the lax standards of literacy and general credulity across the Discworld (worse than here, apparently,) Sek may have started out being Even-Handed; vengeful, perhaps, but equitably so. --Old Dickens 20:30, 27 June 2008 (UTC)