The Literal Commandment
The nakedness of your father's wife's daughter, born to your father, she is your sister, you shall not uncover her nakedness. (Leviticus 18:11)
The literal commandment is that a person must not "uncover the nakedness" of your father's wife's daughter, your sister... either a full sister or (more specifically) a half 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
Just like there are certain animals that were not created to be eaten, there are certain people that were not created to be married (the only context in which conjugal intimacy is permitted). G-d is a G-d of order and structure and we are not to violate the order He puts in place.
In a similar fashion, the Messiah is for salvation. There are no other human beings intended for that purpose in G-d's economy. Trying to find salvation in any other person is like trying to have marital relations with your sister: G-d says "no".
How Messiah Fulfilled
Messiah fulfilled this commandment by not uncovering the nakedness of any of His sisters.
Traditional Observance
The traditional observance of this commandment is found in its literal sense: not being conjugally intimate with one's sister whether a full-sister or half-sister.
Other Notes
We are able to fulfill this commandment today and should not uncover the nakedness of our sisters.
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