AFFINITY. There are several degrees of affinity, wherein marriage was prohibited by the law of Moses: thus the son could not marry his mother, nor his father's wife, Le 18:7, &c. The brother could not marry his sister, whether she were so by the father only, or only by the mother, and much less if she were his sister both by the same father and mother. The grandfather could not marry his granddaughter, either by his son or daughter. No one could marry the daughter of his father's wife; nor the sister of his father or mother; nor the uncle, his niece; nor the aunt, her nephew; nor the nephew, the wife of his uncle by the father's side. The father-in-law could not marry his daughter-in-law; nor the brother the wife of his brother, while living; nor even after the death of his brother, if he left children. If he left no children, the surviving brother was to raise up children to his deceased brother by marrying his widow. It was forbidden to marry the mother and the daughter at one time, or the daughter of the mother's son, or the daughter of her daughter, or two sisters, together.
It is true the patriarchs, before the law, married their sisters, as Abraham married Sarah, who was his father's daughter by another mother; and two sisters together, as Jacob married Rachel and Leah; and their own sisters, both by father and mother, as Seth and Cain. But these cases are not to be proposed as examples; because in some they were authorized by necessity; in others, by custom; and the law as yet was not in being. If some other examples may be found, either before or since the law, the Scripture expressly disapproves of them; as Reuben's incest with Balah, his father's concubine; and the action of Ammon with his sister Tamar; and that of Herod Antipas, who married Herodias, his sister-in-law, his brother Philip's wife, while her husband was yet living; and that which St. Paul reproves and punishes among the Corinthians, 1Co 5:1.