You have read into the Scriptures something that is not there. The Bible does not say that Adam and Eve had only two sons. In fact, it does not say how many children they had. Acording to Jewish tradition, they had 33 sons and 22 daughters. This is not in the Bible but it is certainly believable since Adam lived to the ripe old age of 930. The reason only two sons are mentioned at the beginning of Genesis 4 is because of what happened between then, not because they were the only ones. You must understand that the Bible is a very condensed account of human history only recording the essential information that we need to know. It doesn't really matter to us how many children Adam and Eve had, but it does matter to us that Cain killed his brother and then said, 'Am I my brother's keeper?' Obviously at the beginning of human history brothers and sisters married. This practice was not prohited until the time of Moses. Cain's wife was either a sister, a niece, or a grand niece.