Cain was the first child of Adam and Eve recorded in Scripture (Genesis 4:1). His brothers, Abel (Genesis 4:2) and Seth (Genesis 4:25), were part of the first generation of children ever born on this earth.
Even though only these three males are mentioned by name, Adam and Eve had other children. In Genesis 5:4 a statement sums up the life of Adam and Eve -- "And the days of Adam after he had fathered Seth were eight hundred years. And he fathered sons and daughters." This does not say when they were born. Many could have been born in the 130 years (Genesis 5:3) before Seth was born.
During their lives, Adam and Eve had a number of male and female children. The Jewish historian Josephus wrote that, "The number of Adam's children, as says the old tradition, was thirty-three sons and twenty-three daughters."
The Bible does not tell us how many children were born to Adam and Eve. However, considering their long life spans (Adam lived for 930 years -- Genesis 5:5), it would seem reasonable to suggest there were many! Remember, They were commanded to "Be fruitful, and multiply" (Genesis 1:28).
The Wife
If we now work totally from Scripture, without any personal prejudices or other extra-biblical ideas, then back at the beginning, when there was only the first generation, brothers would have had to have married sisters or there would be no more generations!
We are not told when Cain married or any of the details of other marriages and children, but we can say for certain that some brothers had to marry their sisters at the beginning of human history.
But what about God's Laws?
Many people immediately reject the conclusion that Adam and Eve's sons and daughters married each other by appealing to the law against brother-sister intermarriage. Some say that you cannot marry your relation. Actually, if you don't marry your relation, you don't marry a human! A wife is related to her husband even before they marry because all people are descendants of Adam and Eve -- all are of "one blood." The law forbidding marriage between close relatives was not given until the time of Moses (Leviticus 18-20). Provided marriage was one man to one woman for life (based on Genesis 1 and 2), there was no disobedience to God's law originally when close relatives (even brothers and sisters) married each other.
Remember that Abraham married his half-sister (Genesis 20:12). God blessed this union to produce the Hebrew people through Isaac and Jacob. It was not until some 400 years later that God gave Moses laws that forbade such marriages.
Biological Deformities
Today, brothers and sisters (and half-brothers and half-sisters, etc.) are not permitted by law to marry because their children have an unacceptably high risk of being deformed. The more closely the parents are related, the more likely it is that any offspring will be deformed.
There is a very sound genetic reason for such laws that is easy to understand. Every person has two sets of genes, there being some 130,000 pairs that specify how a person is put together and functions. Each person inherits one gene of each pair from each parent. Unfortunately, genes today contain many mistakes (because of sin and the Curse), and these mistakes show up in a variety of ways. For instance, some people let their hair grow over their ears to hide the fact that one ear is lower than the other -- or perhaps someone's nose is not quite in the middle of his or her face, or someone's jaw is a little out of shape -- and so on. Let's face it, the main reason we call each other normal is because of our common agreement to do so!
The more distantly related parents are, the more likely it is that they will have different mistakes in their genes. Children, inheriting one set of genes from each parent, are likely to end up with pairs of genes containing a maximum of one bad gene in each pair.