"perhaps God kept the adverse genetic effects of inbreeding from happening."
While the first handful of brothers and sisters procreated to produce the next handful of brothers and sisters that would procreate, perhaps God prevented adverse genetic defects for a little while, we don't know... I don't know what's tripping you up.
Have a great day...
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I agree and I found this article that explains a bit more
In Genesis 5:4 we read a statement that sums up the life of Adam and Eve: “After he begot Seth, the days of Adam were eight hundred years;
and he had sons and daughters."
Scripture doesn’t tell us how many children were born to Adam and Eve, but considering their long life spans (Adam lived for 930 years—Genesis 5:5), it would seem logical to suggest there were many. Remember, they were commanded to “be fruitful, and multiply” (Genesis 1:28).
If we now work totally from Scripture, without any personal prejudices or other extrabiblical ideas, then back at the beginning, when there was only the first generation, brothers would have had to marry sisters or there wouldn’t have been any more generations! We’re not told when Cain married or many of the details of other marriages and children, but we can say for certain that Cain’s wife was either his sister or a close relative.
Some say that you can’t marry your relation. Actually, if you don’t marry your relation, you don’t marry a human! A wife is related to her husband before they are married because all people are descendants of Adam and Eve—all are of one blood. This law forbidding close relatives marrying was not given until the time of Moses (Leviticus 18–20). Provided marriage was one man for one woman for life (based on Genesis 1–2), there was no disobedience to God’s law originally (before the time of Moses) when close relatives (even brothers and sisters) married each other.
Remember that Abraham was married to his half-sister (Genesis 20:12). God’s law forbade such marriages, but that was some four hundred years later at the time of Moses.
Each person inherits a set of genes from his or her mother and father. Unfortunately, genes today contain many mistakes (because of sin and the Curse), and these mistakes show up in a variety of ways. The more closely related two people are, the more likely it is that they will have similar mistakes in their genes, inherited from the same parents. Therefore, brother and sister are likely to have similar mistakes in their genetic material. If there were to be a union between these two that produces offspring, children would inherit one set of genes from each of their parents. Because the genes probably have similar mistakes, the mistakes pair together and result in deformities in the children. (Overall, though, the human race is slowly degenerating as mistakes accumulate generation after generation.)
However, this fact of present-day life did not apply to Adam and Eve. When the first two people were created, they were perfect. Everything God made was “very good” (Genesis 1:31). That means their genes were perfect—no mistakes. But when sin entered the world because of Adam (Genesis 3:6), God cursed the world so that the perfect creation then began to degenerate, that is, suffer death and decay (Romans 8:22). Over a long period of time, this degeneration would have resulted in all sorts of mistakes occurring in the genetic material of living things.
But Cain was in the first generation of children ever born. He, as well as his brothers and sisters, would have received virtually no imperfect genes from Adam or Eve, since the effects of sin and the Curse would have been minimal to start with. In that situation, brother and sister could have married (provided it was one man for one woman, which is what marriage is all about, Matthew 19:4–6) without any potential to produce deformed offspring.
By the time of Moses (about 2,500 years later), degenerative mistakes would have accumulated to such an extent in the human race that it would have been necessary for God to bring in the laws forbidding brother-sister (and close relative) marriage (Leviticus 18–20).
https://answersingenesis.org/bible-characters/cain/cains-wife-who-was-she/
