Sister was a title given to all junior members of the Witches of Dathomir society, including the dissident covens known as the Nightsisters.
On the obscure planet Dathomir, the title of "Sister" was used, regardless of actual kinship, as a form of address for a witch, alone or followed by her first name. Sometimes, this title was also applied to offworlders, out of affection or respect—Augwynne Djo once called Princess Leia Organa "Sister Leia".
The first Dathomiri witches were all biological sisters, born to the wayward Jedi Allya, whence the metaphorical use of "Sister" amongst them. Additionally, the sisters referred to their leader as "Mother," further developing the metaphor of the family.
This title first appeared in April 1994 in Dave Wolverton's The Courtship of Princess Leia, where it was extensively used, with variants such as "clan sister", "sisterhood" and "Nightsister".
The use of both "Sister" and "Mother" as forms of address is similar to what happens in the hierarchy of Roman Catholic nuns.
- Book of Sith: Secrets from the Dark Side