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, Lev. xviii, 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 Amnon 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, 1 Cor. v, 1.