From A Wiki of Ice and Fire
The silent sisters is an order of women belonging to the Faith of the Seven. They are sworn to the service of the Stranger and have taken the vows of chastity and silence. They are not to be confused with septas. The silent sisters attend to the dead.
Silent sisters are sometimes referred to as 'Wives of the Stranger' or 'Death’s Handmaidens'. They are shrouded in grey and keep their faces cowled except for their eyes.  It is said that theirs is a serene life, a life of prayer and contemplation and good works. They bring solace to the living and peace to the dead. 
One of the silent sisters' main functions is the collection and handling of the bodies of the dead, preparing them for funerals. There would seem to be more than one accepted methodolgy. For Tywin Lannister the sisters removed his bowels and organs and drained his body of blood, stuffing it with salts and fragrant herbs (some form of embalming, presumably) for its journey back to Casterly Rock. However, since there are many references to a person's bones (rather than their body) being sent for burial, it would appear more usual for all flesh to be stripped from a deceased's bones. Here again, there would appear to be two methods used - beetles and boiling. When Barristan Selmy contemplates sending Prince Quentyn's remains back to Dorne he muses, "We need to strip the flesh from his bones. Beetles, not boiling. The silent sisters would have seen to it back home," implying that beetles are seen as the superior method. A further reference to beetles comes when the skull of Gregor Clegane is stripped of flesh by this method (by Qyburn, rather than the sisters) before being sent to Dorne.
It is said that the silent sisters are always glad to welcome widows. Years ago, during the reign of King Aegon IV Targaryen, the king promised Ser Quentyn Ball a place in the Kingsguard, so Quentyn made his wife join the silent sisters in order to facilitate this. A daughter of Ser Elys Waynwood was seduced by a sellsword, lost her bastard child by him and joined the silent sisters. During the the Great Spring Sickness nearly all of the silent sisters in King's Landing died of the plague.
A Game of Thrones
A Clash of Kings
Theon Greyjoy tells his sister Asha Greyjoy that when he rules the Iron Islands he shall pack her off to the silent sisters. Arya Stark sees silent sisters remove the body of Lord Medger Cerwyn from Harrenhal.
A Storm of Swords
A Feast for Crows
Travelling to Duskendale at the beginning of her quest Brienne of Tarth comes across a band of silent sisters. She asks them if they have seen her sister but they shake their heads. Later Brienne, Ser Illifer and Ser Creighton Longbough come across some poor fellows marching to King's Landing. The poor fellows tell them that even silent sisters have been molested. Creighton remarks that a man would need to be a fool to rape a silent sister or even to lay hands upon one, and he rumours that their female parts are cold and wet as ice.
On Brienne and her companions way to the Quiet Isle talk turns to the silent sisters. When Podrick Payne mentions that he has heard that silent sisters do not have tongues, Septon Meribald smiles and says that mothers have been cowing their daughters with that tale since he was Pod’s age. Meribald explains that,
|“||A vow of silence is an act of contrition, a sacrifice by which we prove our devotion to the Seven Above.||”|
During Jaime Lannister’s vigil over his father’s body the silent sisters come single file down the Stranger’s Steps. Death’s handmaidens are garbed in soft grey, their faces hooded and shawled so only their eyes can be seen. They and the other devout make a circuit of the sept, worshipping at each of the seven altars to honour the seven aspects of the deity. To each god they make a sacrifice and sing a hymn.  When the funeral procession departs King's Landing six silent sisters ride attendance on his bones. At Riverrun Jaime dreams of a woman all in grey, a silent sister. A hood and veil conceal her features but he can see the candles burning in the green pools of her eyes; it is his mother.
Cersei Lannister suggests to Falyse Stokeworth that she join the silent sisters after Bronn ordered her gone from Stokeworth. Cersei then thinks of a better way to get rid of her and gives Falyse to Qyburn. The new High Septon tells Cersei that even silent sisters have been raped. During Cersei's imprisonment three silent sisters hold her down as Septa Scolera strips her bare.
A Dance with Dragons
In Meereen Ser Barristan Selmy considers what is to be done with Quentyn Martell’s body. He thinks that the silent sisters would have seen to it at home, but this is Slaver's Bay and the nearest silent sister is 10,000 leagues away.
|“||Wylla, every time you open your mouth you make me want to send you to the silent sisters.||”|
|“||If I had any sense I’d give you to the silent sisters. They cut the tongues out of girls who talk too much.||”|
|“||A man would need to be a fool to rape a silent sister. Even to lay hands upon one … it’s said they are the Stranger’s wives.||”|
References and Notes
- ↑ 1.0 1.1 A Dance with Dragons, Chapter 65, Cersei.
- ↑ A Clash of Kings, Chapter 39, Catelyn.
- ↑ 3.0 3.1 3.2 A Feast for Crows, Chapter 32, Cersei.
- ↑ 4.0 4.1 A Feast for Crows, Chapter 17, Cersei.
- ↑ 5.0 5.1 A Dance with Dragons, Chapter 70, The Queen's Hand (Barristan IV).
- ↑ 6.0 6.1 A Game of Thrones, Chapter 30, Eddard.
- ↑ The Citadel. Private Meeting Santa Fe, NM (May 4, 2004), So Spake Martin
- ↑ A Feast for Crows, Chapter 41, Alayne.
- ↑ The Sworn Sword.
- ↑ A Clash of Kings, Chapter 24, Theon, p 394.
- ↑ A Clash of Kings, Chapter 30, Arya.
- ↑ 12.0 12.1 A Storm of Swords, Chapter 65, Arya.
- ↑ 13.0 13.1 13.2 13.3 A Feast for Crows, Chapter 4, Brienne.
- ↑ 14.0 14.1 A Feast for Crows, Chapter 28, Cersei.
- ↑ A Feast for Crows, Chapter 31, Brienne.
- ↑ A Feast for Crows, Chapter 8, Jaime.
- ↑ A Feast for Crows, Chapter 16, Jaime.
- ↑ A Feast for Crows, Chapter 44, Jaime.
- ↑ 19.0 19.1 A Dance with Dragons, Chapter 19, Davos.
- ↑ A Dance with Dragons, Chapter 33, Tyrion.