The Sisters of Mercy are the daughters of Chalana Arroy, adopted or otherwise. Malamse, Oronio, and Amprefesne are the three holy Sisters most often called upon, but there are many others. Many living High Healers are often accounted among their numbers and the title “Sister of Mercy” is a common name for High Healers in Dragon Pass.
They live in an invisible nunnery whose exact location or dimensions are unknown. Some claim that it is as large as the world, encompassing all of the cosmos within its walls. This is, however, untrue as proved by the Sage Elad Kramdnil during the reign of Prince Salinarg. He showed that the Sisters are met wherever the old Empire of the Wyrms Friends was dominant, and he was the first to state that they should be considered remnants of that lost empire. This lends credence to the belief that the Empire was not totally corrupt, as such remnants as the Tusk Riders and Delecti would suggest.
The Sisters of Mercy is the subcult most commonly worshiped in Dragon Pass and Prax. It provides the Refine Medicine spell. The center of the subcult is the Nunnery in Dragon Pass.
It is worth keeping in mind that many of the institutions we associate with the Chalana Arroy cult were strongly promoted by the Empire of the Wyrms Friends.Then again, I think many popular assumptions of the EWF are wildly wrong.