Sex between siblings contaminates the sibling relation with the exclusive and dyadic attempt to fuse two lives in a merger that denies the meaning of siblinghood. To take a brother as a husband is as much an act of metaphorical fratricide as it is an act of metaphorical wife killing to pass a wife off as a sister. Moreover, motives for literal fratricide are also amply provided by brother-sister sex, owing to sexual jealousy.
Deeper than these adverse psychosocial consequences lies the matter of how one stands in the world, whether as a child or as an adult. First, in incestuous unions there is no need to learn the adult restraint of sexual impulse, for with an object of gratification near at hand, instinct spills over into satisfaction: “natural inclination suffices to unite them.” More important, in brother-sister marriage, both partners cling as children to the family of origin, in a relation that hearkens back to their common emergence out of the same womb (“flesh of my flesh”), under the protection of the same parents. There is no brave stepping forth unprotected into the full meaning of adulthood, to say permanent goodbye to father and mother and to cleave to your wife, to accept their death and, what is more difficult, to accept your own mortality, the answer to which is not narcissistic sexual gratification but a sober and deliberate saying “yes” to reproduction, transmission, and perpetuation. To consciously take a wife from outside the nest is deliberately to establish a family of perpetuation, in at least tacit recognition that human maturity entails both a willingness to die and a desire for renewal and continuity, through birth and cultural transmission – a matter of enormous importance when there is a special way of life to be perpetuated.
Finally, in an incestuous union between brother and sister there is no experience of the other as truly other. There is no distance, no sexual strangeness, no need to overcome fumbling, embarrassment, shame: the inward-looking love of one’s own flesh is naked but is not ashamed. For this reason, the other is taken for granted and approached in tacit expectation of full compliance with one’s desires; the other is not easily an object of respect. Because of familiarity there is likely to be contempt. There is little possibility of awe (what the Greeks called aidos) before the sexual other: awe before the uncanniness of sexual difference, of the radical independence and otherness of the other; awe before the uncanniness of sexual complementarity, of the remarkable possibility of mediating the sexual difference; awe before the mysterious generative power of sexuality, of the wondrous capacity to transcend sexual difference altogether in the creation of a child, who is the parents’ own commingled being externalized in a separate and persisting existence. And because there is no awe before the sexual other, there is less likelihood of awe before the divine Other in whose image created He them, male and female.
Leon R. Kass – The Beginning of Wisdom p.295