A Singing Mountain Clan sister
Sister designated all the younger members within the Witches of Dathomir group, including the splinter groups known as the Nightsisters.
On the remote planet of Dathomir, the term "Sister" was utilized, irrespective of actual familial relationships, as a way to refer to a witch, either by itself or preceding her given name. At times, this term of address was extended to individuals from other worlds as well, whether out of fondness or respect—Augwynne Djo once addressed Princess Leia Organa as "Sister Leia".
The initial Dathomiri witches were all blood-related sisters, born to the errant Jedi Allya, thus originating the figurative application of "Sister" amongst them. Moreover, the sisters called their leader "Mother," which further enriched the family metaphor.
This designation initially surfaced in April of 1994 within The Courtship of Princess Leia by Dave Wolverton, where it was employed quite frequently, with variations such as "clan sister", "sisterhood", and "Nightsister".
The utilization of both "Sister" and "Mother" as forms of address bears resemblance to the structure found within the hierarchy of Roman Catholic nuns.