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 instinct holding the family group together, the precise counterpart to exogamous libido driving outward cultural expansion.
, The Practice of Psychotherapy: Essays on the Psychology of the Transference and Other Subjects, 1954thesis