Your knowledge is inadequate. Had you read on, you would have seen in the fifth chapter of Genesis that Adam begot Seth, and after that lived on for some 800 years, begetting sons and daughters. Cain very probably married a sister. He could even have married a niece! But that would involve the marriage of a brother and sister at some stage, or indeed of several brothers and sisters. With the cessation of necessity, such close inter-relationship was forbidden. But special conditions naturally prevailed in such special circumstances as the starting of the human race. God exercised a special providence to safeguard the earliest human beings from the evils usually associated with close inter-marriage. And after all, a sister would not be so closely related to Cain as Eve was to Adam. Cain’s wife was not made out of his own rib! Whom Cain married precisely is not mentioned, as not being very important. One book cannot give all the names that have occurred in history, and the Bible gives but a summary outline of chief events.
Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty