A traditional interpretation is one that takes this verse “at face value,” that is, it is, talking about a literal mother/sister relationship. Consider the following from Old Testament scholar Baruch Levine:
18. Do not marry a woman as a rival to her
sister
Hebrew li-tsror reflects the noun tsarah, “rival wife,” which, in turn,
derives from the verb ts-r-r, “to
assail, attack.” In polygamous marriages, the interests of the several wives
inevitably conflicted.
and uncover her nakedness in the other’s
lifetime
The syntax requires clarification. The literal sense is “to uncover her
nakedness (that is, the wife’s sister’s nakedness) in addition to her, during her lifetime (that is, the wife’s
lifetime).” The preposition ʿal in
this statement has the same meaning as ʿal
nashav, “in addition to his wives,” in Genesis 28:9. Marrying two sisters
would create an extremely unhealthy rivalry. The prohibition continues so long
as the first sister remains alive, even if she had been divorced from the man
in question. It is not certain why the text dispenses with the rule of permanent
prohibition in this case. (Baruch A. Levine, Leviticus [JPS
Torah Commentary; Philadelphia: Jewish Publication Society, 1989], 122)
However, it is possible that this text is,
unlike v. 17, more of a general warning against polygamy, not merely marrying a
mother and her sister. Arguing the “likelihood that the traditional
interpretation of Lev. 18:18 is wrong” and that the text “in fact, offers a
general (ethical) prohibition of polygyny” under normal times (cf. Jacob
2:23-30 in the Book of Mormon) Gordon Hugenberger offered the following
justification for such a reading:
The operative expression
אשׁה אל־אחתה (lit., “a woman to
her sister”) is used everywhere in the Old Testament in the distributive sense
of “one to another” and nowhere else refers to literal sisters. Likewise, the
masculine equivalent אישׁ אל-אחיו (lit., “a man to his
brother”) invariably has an analogous distributive sense, “one (man) to
another,” and does not refer, except by coincidence, to literal brothers (i.e.,
Gen. 42:21, 28; Exod. 16:15; 25:20; 37:9; Num. 14:4; Isa. 9:18 [ET 19]; Jer.
13:14; 23:35; 25:26; and Ezek. 24:23). Indeed, had it been the intention of
Lev. 18:18 to prohibit a man from marrying two women who were literal sisters,
it could have been done so with considerably less ambiguity by the use of the
conjunction “and [וִ],” rather than the
preposition “to [אֶל],” that is, woman and
her sister [אשׁה אחתה].” the grammar of
this expression would then be precisely analogous to that of “a woman and her
daughter” [אשׁה ובתה],” the phrase
employed by the author in the immediately preceding verse, where he forbids
sexual relations with a woman and her daughter (cf. also Lev. 20:14). It
appears likely that it was the awareness of this usage which already led the
Zadokites and the Qumran community in the first century B.C., as well as the
much later Karaites, to interpret Lev. 18:18 as an explicit prohibition against
polygyny . . . What is perhaps even more striking, the justification offered in
vs. 18, “to be a rival to her [לצרֹר . . . עליה],” far from
emphasizing the intrinsic perversity of this wrong, is quite general and
applicable to any bigynous marriage . . . the same root, צרר, “to be a rival” or
“to be hostile,” is used in 1 Sam. 1:6 to describe the discordant relationship
between Peninnah and Hannah, who need not have been literal sisters . . . Such
a prohibition against polygyny ought not be dismissed as out of character for
the Holiness Code because of its impossible idealism. Rather, it compares
favourably with a number of other equally idealistic provisions, such as the
prohibition against hatred in Lev. 19:17f.! The fact that Lev. 18:18 and many
other “idealistic” stipulations lack criminal sanctions suggests that these may
have been intentionally ethical, rather than legal norms. Putting this somewhat
differently, Lev. 18:18 can be categorized as a lex imperfecta, a law
which prohibits something without thereby rendering it invalid (reflecting a
society which would have lacked the requisite means of enforcement in any
case). (Gordon P. Hugenberger, Marriage as a Covenant: Biblical Law and
Ethics and Developed from Malachi [Biblical Studies Library; Grand Rapids,
Mich.: Baker Books, 1994], 115-16, 117, 118)
However, the interpretation of v. 17 is
pretty much accepted as one to be taken "at face value":
Thou shalt not
uncover the nakedness of a woman and her daughter, neither shalt thou take her
son's daughter, or her daughter's daughter, to uncover her nakedness; for they
are her near kinswomen: it is wickedness.
Brian
Hales, commenting on this and related texts, noted:
The Torah includes
further restrictions against taking multiple wives, some of which were clearly not
held by the earlier patriarchs. For example, the Israelites were commanded to
not marry sisters or a “woman and her daughter” (Lev. 18:17-18). In contrast,
Jacob married sisters, Rachel and Leah. Abraham’s marriage to Sarah, his
half-sister, would have also violated the law of Moses (Gen. 20:12) (Apparently
Joseph Smith practiced a similar form of plural marriage, being sealed to a
mother and a daughter [Patty Bartlett Sessions and Sylvia Sessions Lyon] and
two sets of sisters [Eliza and Emily Partridge and Sarah and Maria Lawrence]).
Not only was polygamy
allowed and regulated in the Torah, but in at least one situation, it was
actually commanded: “If brethren dwell together, and one of them die, and have
no child, the wife of the dead shall not marry without unto a stranger: her
husband’s brother shall go in unto her, and take her to him to wife, and
perform the duty of an husband’s brother not her” (Deut. 25:5). If a man died
childless, his brother was to marry his widow and raise up children to him.
Thus, if a deceased husband’s brother was previously married, then marrying his
brother’s widow would create a polygamous union. This commandment, known as a “levirate”
law, is recorded in the Torah as being initiated before Moses received the law
known by his name. The situation occurred when Onan was commanded to marry the
widow of his brother Er (Gen. 38:8). Apparently, the living brother could
refuse (Deut. 25:6-10), and later directives have been interpreted as contradictory
by some interpreters (Lev. 18:16; 20:21). However, the levirate law was
apparently still valid in Jesus’s day, as some Sadducees, posing a question on
the resurrection, posited an extreme example of it (Matt. 22:23). (Brian C.
Hales, Joseph Smith’s Polygamy, Volume 3: Theology [Salt Lake City: Greg
Kofford Books, 2013], 34-35)