For the familial connection, please see Sister.
Sister was a title used by female members of some religious orders, such as nuns.
In 1930, Sister Edith Keeler ran a soup-kitchen out of the 21st Street Mission. (TOS: "The City on the Edge of Forever")
In 2376, Dala pretended to be a cleric with the rank of sister as part of a swindle perpetrated on Neelix and Tom Paris. (VOY: "Live Fast and Prosper")
Romulan Admiral Mendak referred to the spy Selok as being a "lost sister" to their people after her complex extraction from the United Federation of Planets. (TNG: "Data's Day")