near to kin:

Heb. remainder of his flesh. Notwithstanding the prohibitions here, it must be evident, that in the infancy of the world, persons very near of kin, and even brothers and sisters, must have joined in matrimonial alliances; and therefore we cannot pronounce them immoral in themselves. But, in these first instances, necessity required it; but when this necessity no longer existed, the thing became inexpedient and improper for:

1. As human nature now is, it is very expedient that those who are so much together in youth, should, by such a restriction be taught to look upon all such intercourse as prohibited and incestuous; for unless such restrictions are made, it would be impossible to prevent the prevalence of very early corruption among young persons. (See Michaelis on the laws of Moses, Art. 108.)
2. 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, or a son and a husband at the same time, and fulfil the duties of both? Impossible.
3. That by intermarrying with other families, relationship and its endearments might be diffused. These prohibitions are, therefore, to be considered so eminently moral obligations as to be observed by all mankind.