Shortly before Israel occupied Canaan.
After God created Adam and Eve, they had sons and daughters [Gen 5:4]. In order for their children to marry and reproduce, it was necessary for brothers to marry their sisters. People have often asked, “Where did Cain get his wife?” The answer is simple, “He married his sister.” There wasn’t anyone else for him to marry.
Therefore, early on in human history it was necessary for close relatives to marry. While we don’t know much about marriage between close relatives during the first 2,000 years of human history, we can presume that close relatives continued to marry because Abram [around 2,000 BC] married his half-sister. Terah is Sarai’s father-in-law according to Gen 11:31 and yet he is also her father according to Gen 20:12. She and Abram had different mothers but the same father, therefore, Abram was Sarai’s half-brother.
When Abraham sent his servant to find a wife for Isaac, God directed him back to Abraham’s family. Isaac married Rebekah, the daughter of Bethuel, who was Nahor and Milcah’s son [Gen 24:15]. Thus, Isaac married his first cousin one removed. Put another way, he married his aunt’s granddaughter.
Jacob married even closer relatives. He married his uncle’s daughters, who were his first cousins. Jacob’s mother was Rebekah. Rebekah’s brother was Laban [Gen 29:10] and Laban’s daughters were Leah and Rachel.
Today, state laws prohibit marriage to a child, grandchild, parent, grandparent, uncle, aunt, niece, nephew, or relative of half blood, such as a half-brother or a half-sister. Several states also prohibit marriage to a first cousin and even to a more distant relative.
These laws are based upon Lev 18 and Lev 20. The prohibited marriages in Leviticus are to your father or mother [18:7], your father’s wife [18:8], your sister [18:9], your grandchild [18:10], your half-sister [18:11], your aunt [18:12-14], your daughter-in-law [18:15], your sister-in-law [18:16, 18], a mother and her daughter or granddaughter [18:17], or a married woman [18:20]. The inhabitants of Canaan had defiled their land by committing these abominations [Lev 18:24-29].
God established these ordinances while Israel was still wandering in the desert. He was getting them ready to inhabit the land of Canaan and he did not want them committing the same abominations that the nations before them had committed. So, up until this time, marriage to a half-sister would have been still allowed as it was in Abram’s case. But after this, it was strictly prohibited.
Notice that these laws of God do not prohibit marriages to first cousins. Thus, when the daughters of Zelophe had married their first cousins to preserve their inheritance within the borders of Manasseh, they were not in violation of the ordinances in Leviticus 18 [Num 36:9-12].
Hope this helps,
Pastor Bevans Welder