If Adam and Eve had two sons, Cain and Abel, then where did all the people come from? Where did Cain find his wife?
Adam and Eve had three sons that were mentioned by name – Cain, Abel and Seth. Even though the first three children were given to us by name, Adam and Eve had other children.
After Seth was born, Adam lived 800 years and had other sons and daughters. Genesis 5:4
Adam lived 800 years after Cain, Abel and Seth were born. You can have a lot of kids in 800 years. God instructed Adam and Eve to have children.
God blessed them and said to them, “Be fruitful and increase in number, fill the earth and subdue it.” Genesis 1:28
Cain then married a sister or possibly a niece. I know that you are thinking that you can’t marry a close relative. You’re right because your children would have too many genetic mistakes. But back in Genesis the genetic makeup was much stronger so that issue was not a problem. But to marry a sister…that’s gross. But back in Genesis when you lived to 900 years you were out of the house before your parents stopped having kids and you didn’t have that brother sister relationship.
The law forbidding close relatives marrying was not given until the time of Moses.
If a man marries his sister, the daughter of either his father or his mother and they have sexual relations, it is a disgrace.