Since the Bible is not being taught very well in too many churches, one of the questions that stymies some people is, "Where did Cain get his wife in Genesis 4:16:17? After all, we only know about Cain and Abel". It's a fair question. However, atheopaths will often use this in a weird, self-refuting way that ends with, "therefore, evolution". They claim that there is no God and the Bible is false, but they selectively cite passages as if they were true to allow God to exist so they can hate him, such as in this image.
The chronology of some of the early Hebrew writings is less linear than our 21st century minds prefer. In Genesis 5:3-4, it said that Adam and Eve had other sons and daughters. As to when they had them, we're not told. Ultimately, its indicates that the sons and daughters married each other. After all, God commanded them to be fruitful and multiply (Gen. 1:28). Incest was expected back then.
Centuries ago when I was in my early twenties, I saw a woman and thought, "Well, hel-lo! Ain't she a babe!" Then I was told that she was my first cousin that I hadn't seen in several years. So much for that idea! If you study on it, you'll realize that incest is not only unlawful, but morally repugnant today. We can't reasonably impose our views on the ancient people who were commanded to reproduce and marry their own kin, can we? Especially since God did not say that it was forbidden for about 2,500 years after creation (Lev. 18:6-18).
From a biblical perspective, everything was created very good (Gen 1:31). There was no genomic devolution yet. That would begin shortly afterward, when man sinned (Rom. 5:12). For that matter, when God said in Genesis 2:17, "In the day you eat of this fruit, you shall surely die". People have said, "Aha! God's a liar (because it's convenient for him to exist right now so I can hate him), because they didn't die! I win the Gotcha Game!" Do your homework, Hoss. The most accurate translation is, "dying you shall die", which means the process of dying had begun, not only for Adam and Eve, but all humanity.
Our genetic clocks are winding down. By the way, ever hear of mitochondrial Eve? Instead of supporting evolution, the science of genetics is supporting the Bible by indicating that humanity can be traced back to one woman. Remember, the Bible tells us that Eve became the mother of all humanity. These facts about genetics really get evolutionists on the prod!
So, incest was encouraged until it became a genetic issue, and at that time, God was likely to be putting a moral repugnance of incest in the hearts of people. Of course, there are people who rebel against God's moral and written laws, and against God himself. all have sinned (Rom. 3:23), and deserve death (Rom. 6:23), but God made reconciliation and salvation possible through Jesus Christ (John 3:16-17, John 1:12).
For further reading on the incest issue by people who are far better than me, see the links above, plus: