He, along with his brothers Abel and Seth (Gen 4:2, 25), were specifically recorded as part of this first generation of children born on the earth. These three boys, however, were NOT the only children born to Adam and Eve.

Genesis 5:4 tells us, “And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters.” Notice that many more children, both boys and girls, were born into Adam and Eve’s home. We are not told when any of these siblings were born but it certainly is likely that many were born in the 130 years before we are told of Seth’s birth. By the way, the Jewish historian Josephus says “The number of Adam’s children, as says the old tradition, was thirty-three sons and twenty-three daughters.”

I believe Cain married his sister. But isn’t that against God’s law prohibiting marriages between brother and sister? The Law forbidding marriages between siblings was not given until long after Abraham and even Joseph died. The Law was given to Moses on mount Sinai and is recorded in Leviticus chapters 18 – 20. In fact, do you remember that Abraham married his half sister? (Genesis 20:12) God blessed this union. Isaac and Jacob married cousins. It was not until at least 400 years later that God gave the Law to Moses forbidding the marriage of close relatives.

Today, brothers and sisters (and even half brothers and sisters, cousins, etc.) are not permitted to marry because their children will have unacceptably high risks of birth defects. This has to do with gene pairs and “mistakes.” Close relations are much more likely to have similar “mistakes.” We are told that the good gene in a pair has a tendency to override the bad so that a deformity (a serious one, anyway) does not occur – that’s good news. However, the higher the number of similar “mistakes” the higher the probability is for a child being born deformed. So we have laws against marrying close relations.

Adam and Eve did not have accumulated genetic mistakes. They were created physically perfect (Genesis 1:31). The effects of sin and the Curse would have been minimal to start with (as it takes time for these copying errors to accumulate). So genetically the danger to children born from parents closely-related would not have been near so problematic. Today, however, there are many “mistakes” in our genes because of sin (Gen 3:16; Romans 5:12; 8:22). 

The idea that God has progressively “managed” the human race by periodically introducing laws is quite Biblical. Man was not allowed to eat meat until after the flood (Genesis 9:3). When Cain killed his brother, God did not require the death penalty. But starting in Genesis 9:5 every murderer was to be put to death. Beginning with the Law given to Moses on Mount Sinai close relations are not to marry. It was not a sin for Adam’s children to marry each other and I believe they did. Cain married one of his sisters.