Thanks for sending your question along to us. I realize that this is a common query from those who have yet to taste and see that the Lord is good. Here is my explanation concerning this subject. It is clear from the biblical text that Adam and Eve’s children intermarried. It is also clear from the biblical record that the gene pool of the original family would not have been burdened, e.g. not yet afflicted with the negative consequences of a newly fallen genotype.
The term incest is generally understood as a prohibition against sexual intercourse between closely related family members. While certain behaviors were not expressly considered sinful because there was no official prohibition against it, e.g. murder was wrong but no official law against was recorded until after Noah’s Flood, Cain murdering his brother Abel had serious consequences.
The one act of father to daughter incest recorded prior to the Mosaic Covenant’s prohibitions against it occurred when Lot was drunk and his daughters seduced him, Gen. 19:30-38. The result was two children Moab and Ben-Ammi who became the leaders of two tribal groups that became the sworn enemies of Israel, the Moabites and the Ammonites. Not a very good outcome.
So, while there was indeed sinful behaviors prior to God’s official decrees outlawing certain behaviors, those penalties were not carried out until the Law was given. As the Bible clearly teaches
12 Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned— 13 for before the law was given, sin was in the world. But sin is not taken into account when there is no law. 14 Nevertheless, death reigned from the time of Adam to the time of Moses, even over those who did not sin by breaking a command, as did Adam, who was a pattern of the one to come. 15 But the gift is not like the trespass. For if the many died by the trespass of the one man, how much more did God's grace and the gift that came by the grace of the one man, Jesus Christ, overflow to the many! 16 Again, the gift of God is not like the result of the one man's sin: The judgment followed one sin and brought condemnation, but the gift followed many trespasses and brought justification. 17 For if, by the trespass of the one man, death reigned through that one man, how much more will those who receive God's abundant provision of grace and of the gift of righteousness reign in life through the one man, Jesus Christ,
18 Consequently, just as the result of one trespass was condemnation for all men, so also the result of one act of righteousness was justification that brings life for all men. Rom. 5:12-18.
It is clear that by the time of the Exodus God codified prohibitions against incest and many other behaviors that were previously not specifically prohibited. God in His infinite wisdom and mercy illustrates the reason for the Law of Moses was to show us right from wrong and indeed show us our need for the Savior, Deut. 18:18; Isaiah 1:18, 53:1-12.
The New Covenant, Jer. 31:31-37, is the covenant God has called us to embrace. It is, unlike the Mosaic Covenant, an unconditional covenant promising that we will know God and that He will forgive us. It is clearly understood to be the replacement for the Mosaic Covenant as noted in the promise itself.
The God of Abraham, Isaac and Jacob is the only true and living God. Unlike the deities of the pagan religions, He is the only One who actually solves the sin problem of mankind. All other false gods do absolutely nothing for their followers. Neither Krishna, nor Buddha, nor Allah have provided any solution to the sin problem of man. Only the God of Israel has proven Himself to be the personification of all love and wisdom, providing an acceptable atonement for the sins of His people and granting forgiveness and everlasting life to “whosoever will may come.”
The Good News is only found in Jesus the Messiah, the God of Abraham, Isaac and Jacob manifested in the flesh to do for us that which we could never have done for ourselves, e.g. keep the Mosaic Covenant perfectly, even becoming our Passover Lamb our exemption from the righteous judgment of God against sin and that always a gift of His amazing grace, Eph. 2:8; I Cor. 5:7.