The Sisterhood of Karn was a female society dedicated to protecting the Sacred Flame, which produced the Elixir of Life.
The Sisterhood was a remnant of the Pythia's power. (NA: Cat's Cradle: Time's Crucible)
The Great Mother of the Sisterhood and her High Priestess Cassandra, from the House of Jade Dreamers, plotted to destroy Rassilon's Foundry along with Lord Provost Tepesh and Lady Ouida from 'The Committee of Three'. Tepesh and Ouida, who were also Vampires, murdered Cassandra before all were killed by the automated sterilization of the Foundry. (BFA: Zagreus)
The Sisterhood entered into the Pact of Rassilon for protection under Gallifrey they also fought against Morbius when he attempted to invade Karn. (PDA: Warmonger)
When the Sacred Flame appeared to be dying, the Sisterhood suspected the Doctor of trying to steal the remaining Elixir, but later helped him defeat Morbius. (DW: The Brain of Morbius)
After Romana became President of Gallifrey, she opened diplomatic relations with the Sisterhood. (NA: Lungbarrow)
The Sisterhood are capable of teleportation using the Sacred Flame. (BFA: Sisters of the Flame/The Vengeance of Morbius )
They later tried to prevent Morbius' resurrection. (BFA: Sisters of the Flame/The Vengeance of Morbius )