Some people think that Adam and Eve only had 3 sons. The three are Abel, Cain, and Seth. However, we should notice that the Bible never said that Adam and Eve "ONLY" had three sons.
On the contrary, the Bible tells us that the couple actually had sons and daughters.
"And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters." -Genesis 5:4
Apparently, Cain would have married one of his sisters.