As I was looking into this concept, I was mildly surprised to find that it is not at all an uncommon theme in myth, literature, and cinema. Incest among siblings may be particularly relevant among fraternal twins of the opposite sex, where one could claim the bond is stronger. In Balinese culture, twins were forced to marry because of the assumption they had sex in the womb. Native American Mohave culture regards male-female twins as having been married in heaven (http://en.wikipedia.org/wiki/Twincest).
As incest is taboo in most cultures (though the definition varies), its’ revelation is always shocking and monstrous. It destabilizes social relations once it is no longer a secret. In the film Cruel Intentions, incest takes on a more sinister turn as it manifests a rivalry between two step-siblings (Ryan Philippe and Sarah Michelle Gellar). In Clueless, it is almost cute. In The Royal Tenebaums, Richie Tenebaum (Luke Wilson) is in love with his adopted sister Margot (Gwyneth Paltrow). We tentatively accept it in these cases, though finding it mildly scandalous, because of the lack of a blood relation.
Incest between siblings is also a fairly common concept in Japanese animation. In Revolutionary Girl Utena, members of an elite high school circle compete for possession of the mythical “Rose Bride.” In her incarnation as a high school student, the Rose Bride-- Anthy Himemiya-- engages in a semi-consensual incestuous relationship with her sexy, but dominating older brother Akio. He himself has a literally split personality, which expresses itself as two separate characters during the series. Anthy is torn between her long-standing duty to serve and to protect him and her unconventional love for her friend Utena.
The most well-known implication of an incestuous pairing is probably in Star Wars, where twins Luke and Leia develop a romantic attraction to each other prior to realizing that they are related. While there is definite sexual tension between them and even a kiss, it is replaced with filial affection once they both know the truth, and Leia herself becomes attracted to Han Solo before this fact is revealed, thereby conveniently avoiding tragedy.
In A.S. Byatt’s Angels and Insects, a Victorian naturalist marries the beautiful Eugenia of a rich family that has been his patron only to discover that she has been involved in a long-standing incestuous relationship with her brother which goes further than just incest, but also to inbreeding. All of her children are discovered to be those of her brother’s and Eugenia reveals that though she had tried and wanted to be normal and good, she was unable to put a stop to it.
Of course, all these examples invite the question—why? While incest implies a breakdown of normal, human relations at the most primal level, at the same time, it is primal in and of itself, and falls comfortably into Freud’s concept of the uncanny—“that class of the frightening which leads back to what is known and long familiar.” ("The Uncanny", p. 200). We are all attracted towards the familiar. Daphne du Maurier’s use of incest in her writing is related to the concept of “home” or the attraction towards that which is familiar, unconscious or not.
Perhaps the most compelling, clever, and layered use of it as a thematic device belongs to The House of Yes (Mark Waters, 1997), a film based on a play by Wendy MacLeod. The film adaptation is at once a gothic horror spoof, dark comedy, and historical reenactment—but mostly I feel that it operates with the precision of a Greek tragedy and the gut-wrenching emotional impact of Racine. The film stars Parker Posey as Jacqueline Pascal or “Jackie-O” and Josh Hamilton as Marty Pascal, reunited after an indeterminate period at Thanksgiving (during an unseasonable hurricane). Marty has brought home a fiancé, whom Jackie is meeting for the first time. During the course of the night, it is revealed that Marty and Jackie had been having sex with each other for a very long time. It probably began at age 14 with an Ides of March party in which they attended as John F. Kennedy and Jackie Onassis, as Posey's character relates, “in a pink Chanel suit, pillbox hat, and blood on my dress.” Posey, who herself has a twin brother twin in real life, owns the film. Her performance is visceral, frightening, and poignant.
On the day of the Ides of March party, the twins had invented a ritualistic game --a reenactment of the Kennedy assassination (in which Posey's character as Jackie Kennedy doubles as the killer), to enable them to break boundaries and to make love to each other (http://wendymacleod.com/plays/house_of_yes.shtml). Fraternal, opposite-sex twins are as close to each other as any man and woman can be. They are closer to each other than any friend or future significant other can be. That is somewhat tragic, because other women and men will always be sidelined (I’m not sure if I could be with a man who had a twin sister—I’d know I could not, nor would I want to, compete with that). This is of course not to say that all twins are incestuous because evidently the majority of them are not, but more to put the question out there: If they are already that close to each other, is sex between them really that shocking or unnatural? The twins’ mother tells Marty’s fiancé, “All I know for sure is that Marty and Jackie belong to each other.”
In the myth of Narcissus, our mythical Greek character is unable to come to terms with his own reflection, and drowns in it. Freud defines Narcissism as “unbounded self-love . . . which dominates the mind of the child and of the primitive man” (The Uncanny, p. 256). Jackie tells her twin, “Now you and I Marty, have a great deal in common: parents, DNA, bone structure.” Younger brother Anthony feigns being appalled by the revelation of their sexual liaison (though it’s clear that he himself always knew) and exclaims, “It’s like fucking a mirror!” In incestuous narcissism, according to Gail Finney, “erotic energy is transferred from the narcissistic individual to the object most like himself, his sibling.” (“Self-Reflexive Siblings: Incest as Narcissism in Tieck, Wagner, and Thomas Mann. The German Quarterly, Vol. 56, No. 2, p. 243). Finney’s analysis concerns siblings Siegmund and Sieglinde in Richard Wagner’s “The Valkyrie,” in which the two find each other after a long period of separation. The attraction develops as they recognize the similarities in each other and they end up having sex. Sieglinde gives birth to Siegfreid, an iconic figure in Norse mythology. Finney writes, “Sieglinde realizes that only upon seeing Siegmund did she become fully conscious of herself. . . Siegmund is her mirror” (p. 247). In The House of Yes, Marty attempts to argue with his fiancé that sleeping with his sister is analogous to sleeping with himself.
Towards the end of the film, Jackie (in particular) and Marty both began to confuse the historical memory of Kennedy's death with their reenactment game, and with the departure of their own father 20 years ago on the same day. Jackie asks Marty to reenact the game with her one last time, promising to then restore him to the real of normalcy. The tragedy is that ultimately, Marty and Jackie’s relationship presents a crash of their world with the “normal” world and there is no room for reconciliation. Marty and Jackie are in love with each other and it’s not a love that can exist in the world outside of their own house. At the end, they are both aware of and confront and embrace this truth.