Leviticus 18:6

Any that is near of kin - בשרו שאר כל col shear besaro , any remnant of his flesh, i.e., to any particularly allied to his own family, the prohibited degrees in which are specified from the 7th to the 17th verse ( Leviticus 18:7-17 ;) inclusive. Notwithstanding the prohibitions here, it must be evident that in the infancy of the world, persons very near of kin must have been joined in matrimonial alliances; and that even brothers must have matched with their own sisters. This must have been the case in the family of Adam. In these first instances necessity required this; when this necessity no longer existed, the thing became inexpedient and improper for two reasons:

1. That the duties owing by nature to relatives might not be confounded with those of a social or political kind; for could a man be a brother and a husband, a son and a husband, at the same time, and fulfill the duties of both? Impossible.

2. That by intermarrying with other families, the bonds of social compact might be strengthened and extended, so that the love of our neighbor, etc., might at once be felt to be not only a maxim of sound policy, but also a very practicable and easy duty; and thus feuds, divisions, and wars be prevented.