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, 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).
One might wonder why these weaknesses set in. It is obvious Cain did not care about God's law, else he would not have murdered his brother (Genesis 4:8; Exodus 20:13). If he disobeyed God in this matter, he likely disobeyed God in matters of diet (Leviticus 11, Deuteronomy 14), and ate whatever pleased him.
Certain animals simply are NOT FIT for human food and because of the nature of their flesh slowly, but certainly, break down the human system bringing on illness. Ingestion of the genetic code of such animals would obviously cause mutation of genes in the human body.
So, although it is easy to say who became the wife of Adam and Eve's first two children, it helps to know why God allowed intra-family marriages before the flood, yet outlawed them after. Thank you for submitting your question about who did Cain and Abel marry.