Adam was the first man (1 Cor. 15:45) and Eve is the mother of all living (Gen. 3:20). Genesis 5:4 tells us that Adam and Eve had both sons and daughters besides Cain, Abel and Seth. So Cain married one of his sisters.
“Now wait a minute,” someone might say, “doesn’t the Bible forbid brothers and sisters to marry?” Yes it does, but that law came many, many years later (in Lev. 18-20). During the time of Cain it was okay to marry siblings. Here’s why:
Our genes have mistakes in them. When people who aren’t related have children, their offspring inherit different bad genes. But when relatives have children, there is a much greater chance that the children will inherit the same bad genes, resulting in an increased likelihood that they will have birth defects.
But before the fall of man, Adam and Eve had no bad genes (Gen. 1:31). After the fall, Cain and his brothers and sisters had bad genes, but not many. As time went on, the amount of bad genes humans were carrying had increased to the point at which God had to forbid relatives from marrying and having children.