Incest, as an endogamous relationship, is an expression of the libido which serves to hold the family together. One could therefore define it as ‘kinship libido,’ a kind of instinct which, like a sheep-dog, keeps the family group intact.
Jung’s canonical definition of kinship libido as the endogamous, family-binding instinct dialectically opposed to the exogamous tendency that drives union with strangers.
, The Practice of Psychotherapy: Essays on the Psychology of the Transference and Other Subjects, 1954thesis