Not much is know about The Sisterhood except they are not like normal Vampires, and have existed for around 200 years. They are very powerful and follow one leader, and when that leader is killed, their fealty is switched to whomever killed the previous leader.
They used to be ordinary humans until they are bitten by a Sister. It is implied in Black Days that they 'serve the lord', and thus may be Vampire Hunters themselves, although this may be in reference to something else, for example becoming a nun. They are shown as somewhat superior to most Vampires, both as Dmitri fears them and needs their help to open the crypt to The Elder.
In Wakey, Wakey , one of the warriors fighting Scarlett praises her combat skills, and offers her the bite to become a Sister. Scarlett refuses, and is moments later is bitten by one of the Sisters on her neck, that sister then turns back to human. The Sisterhood is shown to have contempt for humans, and the Sister who has just been turned into a human is called a ''pig human'' and killed.