The Sisters of Silence were the military wing of the Adeptus Astra Telepathica (The Imperium's Psyker division), created by Malcador to assist in dealing with rogue psykers, though they are also extremely effective against daemons. They, along with the Adeptus Custodes, are one of the few forces to serve the Emperor directly. The Sisters were an all-female force of psychic blanks (meaning that their very presence actively suppressed psychic powers), and took vows of silence. They were present in large numbers during the Siege of Prospero, and the War in the Webway.
After the Horus Heresy, the Sisters shrank as an organization; many were folded into the League of Black Ships and disappeared from the battlefield. The rest, who disagreed with the new direction the Imperium was taking under the High Lords' rule, disbanded as an official organization, but continued to recruit members and serve the Emperor's ideals. They have since fought during the War of the Beast and the Gathering Storm, where they've returned to the forefront to meet the growing tide of Chaos.
The Sisters of Silence do not appear until Wamuudes' Vox Logs, where a trio of Oblivion Knights accompany him into the Emperor's personal sewer system to clear out psychic abominations living within the pipes and tunnels. They otherwise only have very passing references in the series, leaving their level of activity within and outside the Imperial Palace somewhat vague.