Family Fusion

 "This understanding of marriage is then confirmed by Leviticus 18:15, “You shall not uncover the nakedness of your daughter-in-law (כַּלָּֽתְךָ֖); she is your son’s wife, you shall not uncover her nakedness.” Because a man’s wife has the status of an adopted sister, that man’s father has to treat the woman as if she were his own “flesh of flesh” daughter. In other words, through marriage, husbands and wives share the same parents and can thus truly be considered adopted siblings.This sheds light on a strange detail that pervades the Song of Songs, namely, the fact that Solomon keeps referring to his wife as his sister. Over and over again Solomon pours out his love for the woman he calls “my sister, my bride” (Song 4:9-10, 12, 5:1), even though we don’t have a record of Solomon ever marrying one of his biological sisters. In light of Scripture’s teaching on adoption, it’s most likely that Solomon’s wife could be called his “sister” because of her marriage-adoption into Solomon’s family. This is further supported by the fact that the word translated as “bride” throughout the Song is “כַּלָּ֔ה,” which is the very word used in Leviticus 18:15 to reference daughter-in-laws. This strongly suggests that the Levitical understanding of marriage-adoption is at play here, demonstrating that, through marriage, the woman of the Song became the daughter of David, and the sister-bride of Solomon.

All of this is crucial to understanding the only passage relevant to polygamy in Leviticus 18, “And you shall not take a woman as a rival wife to her sister, uncovering her nakedness while her sister is still alive” (Lev. 18:18). Although the popular interpretation of this verse is that it only condemns polygamy with the biological sister of one’s wife, we now know that it’s doing much more than this. Because marriage entails the status of an adopted sibling, if a man has two wives then these wives would always be sisters by virtue of being married to the same man, and thereby sharing his father. Leviticus 18:18 is thus a condemnation of polygamy as such, which is why it refers to multiple sister-brides as “rivals” (צָרַר). An almost identical word is used in 1 Samuel 1:6 to describe how Peninnah and Hannah were “rivals” (צָרָה) due to their mutual marriage to Elkanah, however it’s never implied that they were biological sisters. Instead, the reason two sister-brides are rivals isn’t due to their biological lineage, but rather their marriage-adoption to the same man." - Codex Justinianeus,  ancientinsights.wordpress.com

Where else can we see this dynamic of separate families becoming as one family?  We see it in the geneology of Christ. There is a recorded geneology of Christ through Joseph and though Joseph gave no genetic makeup to the creation of Christ, He is still regarded as if he hadand the account is listed in Matthew vs the genealogy through Mary in Luke. This fusion of seperate families becoming as one family is a fascinating dynamic and bolsters the claim made regarding the prohibition against polygamy.  Furthermore we also see the commands if a widowed woman has no children before her husband dies then the brother is to give her a son that there may be an heir and the name of the brother not be cut off before God. 

We can see in elements of the 

Genesis 38:8 8 Then Judah said to Onan, “Go in to your brother’s wife, and perform your duty as a brother-in-law to her, and raise up offspring for your brother.” 

There appears to be a transitive property that occurs where the child is as the fallen brothers offspring, even though the fallen brother is not the one that passed his seed to the wife. This would further back the case in leviticus 18 where the married addition becomes as the natural- and by extension of the principle be a point against polygamy as a man is not to take another woman as rival to his wife since she would become her sister. 

We see this same dynamic in passages like Ephesians 2 where it is stated that we were FORMERLY gentiles but have now become adoptive sons.