Brothers of the Sith was the title used by the so-called "twins" Picaroon C. Boodle and Gorc, while serving as two of Jerec's Seven Dark Jedi. They were sometimes also called the Twins.
Pic and Gorc were not siblings, and neither were they Sith. They were, in fact, the result of an experiment in Sith alchemy carried out by Cronal under orders from Emperor Palpatine. Cronal mutated a Kowakian monkey-lizard and a Gamorrean by exchanging genes between them, artifically making them genetic "brothers". As a result of the mutation, the two Sithspawn shared a Force life-bond that resulted in the ability of each to feel the others' location, emotional state, and senses, including even pain.
When seen together, the Twins seemed like a comical pair, owing to their difference in intelligence, talkativeness, size, height, strength and mass. In practice this meant that they covered a wide range of abilities, complementing each other. Between the two, the more intelligent Pic had the role of the "big brother," caring also for Boc.
The pair went on to serve Inquisitor Jerec as two of his Seven Dark Jedi and participated in the execution of Qu Rahn's team and the deactivation of 8t88. When Jerec and his Dark Jedi faced Kyle Katarn, Gorc was killed and the bond that Pic shared with his "brother" caused the living twin to feel as if half of him had died. Pic accordingly fell into a mad frenzy until he joined his other half in death.
Individually, Pic's speed and cunning alongside Gorc's raw power went hand-in-hand during battles. The two always worked and battled as a team, covering all angles and weaknesses making their combination a deadly one. Pic often rendered himself invisible while Gorc often choked his opponents telekinetically, immobilizing them. As Rahn described, "Pic is the energy and Gorc the counter."
The Twins shared awareness and some telepathy; each individual knew the other's location, senses and emotional state, and when they were in close proximity, they individually became more powerful. The side-effect was that they also shared pain, injuries, and corruption by the dark side. When Gorc was killed, the surviving part of the pair felt like losing his half life.
In Star Wars: Jedi Knight: Dark Forces II, the level in which the player must face Gorc and Pic (Level number 11) is titled "Brothers of the Sith," and Qu Rahn describes the two as twins. Details of their relationship are expanded on by Wizards of the Coast.