The Bible says that Adam and Eve had Cain and Abel and then Cain went somewhere and found a wife. Where did she come from? Genesis 3 verse 20 says that Eve was the mother of all living and Genesis 5 verse 4 says that Adam and Eve had other sons and daughters. So Cain would have married one of his sisters. But this answer raises other questions. Remember that the law against close intermarriage did not come until the time of Moses. Biological problems in their children were also not a problem at that time since the genetic mutations that cause problems increase over time. In the beginning God created the genes with no mutations. For more details go to Creation.com and search on 'Cain's wife'.