GLEANINGS — April 28, A.D. 2013                                                                                                               "Where did Cain Get his Wife?"

Let's approach that question with some hard Biblical facts.

Adam was the first human being, his wife Eve was formed by God from his body, and from this couple the entire human race has descended. Genesis 2:21-13; 3:20, Acts 17:26. 
Three sons of Adam and Eve are named in Gen 4. They are Cain, Abel, and Seth. Sometimes overlooked is the statement in Genesis 5:4 – "After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters." 
The first commandment God gave to Adam and Eve, before the Fall, was "Be fruitful and multiply." The as yet sinless couple, created with a sexual nature, began to obey that first commandment right away, within the divine, holy, and righteous institution of marriage – and that with pleasure, as it is ultimately pleasurable to obey God in all things. 
The point is that though only three are named, Adam and Eve had other children, and probably very many; Seth was born when Adam and Eve were 130 years old. Cain and Abel were Seth's younger brothers, and these three brothers had (unnamed) sisters.
Considering the longevity and great fertility of the early humans, it is most reasonable to conclude that the human race did indeed multiply very quickly. Per Genesis 4:15, when Cain killed Abel, God set a mark on Cain, "lest anyone finding him should kill him." This implies that there was already a human population more numerous than merely Adam, Eve, Cain, and the lately murdered Abel. 
In later times, God forbade marriages between those closely related, but at the beginning this was not the case. It would take many generations of human procreation before genetic debilitation would arise in the offspring of a man and woman of near kinship. At the beginning, pre-existing relations such as first cousins, uncle and niece,  nephew and aunt, and even brother and sister did not make marriage unlawful or unwise for a prospective couple.
No doubt these considerations seem quite alien to our 21st century A.D. minds. One reason is that they are radically contrary to the evolutionistic mythology so ingrained in our society: that our earliest ancestors were stooped, grunting, ape-like creatures barely above the animals, with reason only beginning to dawn beneath their low and hairy brows. 
Expressly to the contrary, as the infallibly reliable geneologies of Genesis and the Holy Spirit's comments within them reveal, our earliest ancestors enjoyed lifespans measured in centuries, not decades. They were robust, intelligent, and creative: creatures made in the image of the one God Whom they worshiped until apostasy begot such errors as polytheistic idolatry and animism, along with all the violent wickedness that grieved God's heart and brought down His judgment in the form of the great Flood which swept away the world that then was.
 Cain's wife was also his sister - the daughter of another couple who were also brother and sister to Abel, Seth, and himself – or possibly his niece.