Sororicide

Sororicide (from Latin soror "sister" + -cide, from caedere "to cut, to kill") is the act of killing one's own sister.

There are a number of examples of sororicide and fratricide (the killing of one's brother) in adolescents, even pre-adolescents, where sibling rivalry and resulting physical aggression can get out of hand and lead to the death of one of them, particularly when a potent weapon is available or one is significantly older than the other and misjudges his/her own strength.

Sororicide is similar to child murder (the killing of an unrelated child), infanticide (killing of an infant under the age of one year), filicide (the killing of a child by his or her parent), and patricide and matricide (the killing of a father or mother respectively by his or her child).

Known or suspected sororicides

 * The story of the Horatii (which may or may not be a legend) involves a sororicide: after a set of Roman triplets, the Horatii, defeat a set of Alba Longan triplets, the Curiatii, in single combat to keep Rome from Alba Longan rule, the Horatii's sister was killed by the surviving Horatius for mourning the death of one of the Curiatii, to whom she was engaged. The Roman courts condemned the Horatius to death for the murder despite his service, and it was only his father's appeal to the people that saved him.
 * Berenice IV of Egypt is believed to have poisoned her sister Cleopatra VI Tryphaena in 57 BC. She was later beheaded on the orders of her father, Ptolemy XII.
 * Cleopatra of Egypt requested the execution of her sister, Arsinoe IV, which was carried out under the orders of her lover Mark Antony in 41 BC.
 * Roman Emperor Caligula, according to historian Suetonius, killed his sister Drusilla after learning that she was pregnant with his child in 38 AD. Most historians now believe that she probably died of fever.
 * Roman Emperor Commodus ordered his older sister Lucilla to be put to death in 182 AD, after she was implicated in plots with members of the Senate to overthrow him.
 * Dipendra of Nepal (1971-2001) massacred much of his family at a royal dinner on June 1, 2001, including his father, mother, brother, and sister Princess Shruti. He also shot himself, technically becoming King of Nepal for three days until his death on June 4, when he was succeeded by his uncle Gyanendra.
 * Ronald DeFeo, Jr. shot his two sisters, Allison and Dawn, in 1974. Their murders became the inspiration for the Amityville Horror books and films.
 * Canadian serial killers Karla Homolka and Paul Bernardo raped, then accidentally killed Karla's sister Tammy (1990).
 * Yuki Muto murdered his sister Azumi Muto on December 30, 2006 in Japan.

Mythology

 * According to Roman mythology, one of the Horatii killed his sister after she mourned an enemy he had slain, who had happened to be her fiancé.

Literature

 * In William Shakespeare's King Lear, Goneril poisons her sister Regan in their struggle for power.
 * In the horror manga "Death Panda" by Waita Uziga, Suzuran, a former shrine maiden turned demonness, rapes and kills her sister Yuuki with the assistance of the Death Panda in order to fill her own desire for lust, human flesh, and violence.
 * In the manga series Reiko the Zombie Shop, The main antagonist Riruka attempts to kill her sister Reiko out of jealousy over Reiko gaining the responsibility of inheriting their family's title of the "Zombie Shop." Also, the serial killer Saki Yurikawa forces younger girls to be her sister and brutally murders them when they refuse.

Films
in the 2002 movie Chicago Velma kills her husband and sister after she sees them doing the Spread Eagle.
 * In the Halloween film series, serial killer Michael Myers relentlessly pursues and slays both his older sister, Judith, and his younger sister, Laurie.
 * In the 1962 movie What Ever Happened to Baby Jane? (as well as the 1991 TV movie What Ever Happened to Baby Jane?), sisters Blanche and Jane mistreat and attempt to kill each other out of jealousy and hatred.
 * In the anime Blood+, Chiropterans have a biological hive system. Two twin Queens are always born from cocoons in one of the preceding Queen's womb. When the blood of one Queen mixes with her sister's, it violently crystallizes. In Blood+, Saya kills her twin sister Diva in a 123 year-old war between them.
 * In Fatal Frame II: Crimson Butterfly, the village forces twin sisters to partake in a ritual in which one sister strangles the other. In the canonical 'bad ending', Mio Amakura does indeed strangle her twin sister Mayu to death. (The other documented instance of sororicide is when Azami Kiryu kills her sister Akane.)
 * In the anime Death Note, Light Yagami considers killing his sister, Sayu when she ends up being captured by Mello and his gang in an attempt to retrieve the titular Death Note.
 * In the anime Code Geass, Lelouch Lamperouge kills his half-sister Euphemia li Britannia in order to stop a massacre that he accidentally ordered her to start. Also, Schineizel guns down his sister Cornelia after revealing his plans to rule the world as a god.
 * In the horror film Catacombs, a deranged Victoria kills her sister Carolyn in a rage after being yelled at over the accidental murder of Jean-Michael.
 * In the TV show Family Guy, Chris remembers in Let's Go to The Hop that Meg, his older sister, strangled their other sister, but this could just have been a dream.
 * In the Angel Sanctuary OVA, Rosiel is shown happily saying he will murder Alexiel, who is currently inhabiting Setsuna's body, if she does not return his love.

Song

 * In Child Ballad #10, "The Twa Sisters", or "The Dreadful Wind and Rain" the older sister murders the younger sister over the love of a man, and the younger sister's bones are found by a wandering musician who makes an instrument (either a harp or a fiddle, depending on the version) out of them and strings it with her hair. The instrument then tells the story of how she died, usually resulting in a gruesome death for the older sister.

Games

 * In the God of War video game series, Kratos accidentally murders Athena, where he learns she's Kratos's half-sister because it was revealed that Zeus is his birth father.
 * In No More Heroes, Travis Touchdown kills his half-sister Jeane to avenge the death of his family, an event Jeane herself was responsible for.
 * In Phoenix Wright: Ace Attorney - Trials and Tribulations, Dahlia Hawthorne kills her stepsister Valerie Hawthorne in the fourth case of the game when Valerie threatened to expose the truth behind a fake kidnapping that Dahlia and her boyfriend plotted five years ago in order to steal a rare diamond. Also, the second case of the serie's first game sees Maya Fey, assistant of the titular protagonist, accused of Sororicide.
 * In one of the endings of the video game, Trapt, Allura kills her half-sister, Rachel, for the final sacrifice to revive Malphas after becoming possessed by the aforementioned demon. Also, Finnegan kills his older sister Ada in an act of betrayal after being corrupted by the desire to obtain Allura's power for himself.
 * In the Tekken series, Nina and Anna Williams are assassins and sisters who have an immensely bitter, sororicidal sibling rivalry.
 * In Fable and Fable: The Lost Chapters, after defeating Jack of Blades, the Hero is faced with the option of killing his sister Theresa and receiving the Sword of Aeons, or tossing the sword into the vortex and ridding the world of its evil forever.