“The Sisters” is a shorthand term for a group of female agents apparently intent on bringing about some nasty outcome to the Five Kingdoms. They are not human (some, maybe all, are hags), and their plot has been in the works for at least two hundred years. Whether they are actually blood relatives is unknown. There are at least four Sisters, and there may have been more.
Their efforts are directed by “Mother,” who does not appear to be manifest on this plane. “Mother” communicates (apparently sporadically) with “Lady Corruptress,” who appears to hold the position of seniority. There is no further information as to the identity or whereabouts of “Lady Corruptress” at present.
The Sisters appear to have been involved in causing the collapse of the Corovian empire and fomenting The Cleansing, and reason exists to believe they are initiating the next stage of their long-term plan. It is plausible that this next stage requires some long lost artefact, and that one of the Sisters commissioned the creation of the Treldor Historic Society for the purpose of recovering it (though the Sisters have also engaged other organizations, including the Dawn and the Iron Alliance, to that end.
The woman calling herself “Sister” presents a conundrum, as she does not appear to be working toward the group’s goals, and if she is, her chosen form of identification would seem a little obvious.