Boyd’s Bible Dictionary:
Relation by marriage and not by blood or birth (1 Kings 3:1). For preventive degrees see Leviticus 18:6-17, and MARRIAGE.
Concise Bible Dictionary:
In relationship by marriage, the principal point of interest is the degrees of relationship within which marriage was forbidden. Thus a woman must not marry two brothers, unless her husband had died without issue, when the brother was required to raise up seed to the deceased. Apparently a man might by the law of Moses marry his deceased wife’s sister; but not the second in the lifetime of the first(Lev. 18:18), though Jacob before the Law was given had two sisters for wives at the same time. Near relatives may not marry (Lev. 18). Many codes respecting the degrees of affinity have been made by nations, more or less in agreement with the law of Moses. See MARRIAGE.