Who did Cain and Abel marry?
Question: Who exactly did Cain and Abel marry since women, other than Eve, are not mentioned in the early part of Genesis?
Answer: The debate over who became the wife of man’s first two children has gone on for centuries. The short answer is that the brothers had to marry their sisters. Man was, at first, only two people, Adam and Eve. The couple's first three children were Cain, Abel and Seth: "After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters." (Genesis 5:4)
How many sisters were potentially available for Cain and Abel to marry? The Bible does not reveal exactly how many children Adam and Eve produced. It is safe to assume, however, that since Adam lived 930 years, he had PLENTY of time to take full advantage of God's command to procreate and fill the earth!
A footnote in the writings of the Jewish historian Flavius Josephus (Antiquities of the Jews, Book 1, Chapter 2) states that there existed an old tradition which said Adam had THIRTY-THREE sons (which included Cain and his brother) and TWENTY-ONE daughters! Adam and Eve's creation was perfect in every way. Their children would also be born physically and genetically perfect. For a time after humans were created men could (and had to) marry one of their sisters - without the genetic problems that would later arise.
God, sometime after the flood, began commanding that near-relative marriages should not occur (Leviticus 18:6). If Adam and Eve received the same command God gave to Israel, and they and their children all obeyed it, man would have quickly died out. God, therefore, temporarily allowed intra-family marriages to increase his earthly family.
By the time of the flood, severe genetic weaknesses had likely set in, making close-kin marriages impractical, and greatly reducing the average life span of man after the flood. This would prevent another population explosion of the pre-flood magnitude. (Mathematicians have calculated a billion people may have died in the flood.).