The Literal Commandment
You shall not marry a woman in addition to her sister as a rival while she is alive, to uncover her nakedness. (Leviticus 18:18)
The literal commandment is that a person must not marry or "uncover the nakedness" of your wife's sister.
"To uncover the nakedness of" is a Biblical idiom for "having sexual relations with".
Messiah Says
Messiah implicitly affirmed this commandment when He spoke about the Law:
"Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished. Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven. (Matthew 5:17-19)
Pictures of Messiah
Marriage is the only context in which sexual intimacy is recognized and permitted. This commandment explicitly links marriage and sexual intimacy ("uncover her nakedness"). In order to "uncover the nakedness" of a woman and her sister, a man would have to be married to both of them. This means his attention and affections would have to be split between two women which would likely result in jealousy and rivalry. Scripture uses the physical reality of a relationship between a man and his bride to picture the reality of the relationship between Messiah and His spiritual bride.
The marriage of a man to a woman and her sister would provide an incorrect picture of a spiritual truth regarding Messiah and therefore is forbidden.
How Messiah Fulfilled
Messiah was unable to fulfill this commandment because He was not literally married to any woman and could not have possibly married a non-existent woman's sister.
Traditional Observance
The traditional observance of this commandment is found in its literal sense: not being conjugally intimate with your wife's sister.
Other Notes
We are able to fulfill this commandment today and should not uncover the nakedness of our wife's sister.
Although Jacob married both Leah and Rachel (sisters), Scripture never charges him with violating this commandment. Either the commandment was not in place/in effect during his lifetime or there were some mitigating circumstances that superseded this commandment.
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