With whom did Cain and Seth have children since the Bible doesn’t mention any other women besides Eve?

Adam named his wife “Eve” because “she was the mother of all living” (Gen. 3:20).  Scripture states that in addition to Cain, Abel, and Seth, Adam and Eve “had other sons and daughters” (Gen. 5:4).  God had told them to “be fruitful and multiply and fill the earth…” (Gen. 1:28).  Since Adam lived for a total of 930 years, it is clear that he and Eve were able to spend several centuries fulfilling that command to fill the earth with their children. 

Thus, Cain and Seth, as well as the rest of Adam and Eve’s immediate children, could in turn have children by marrying their sisters.  This would be the origin of the wife Scripture mentions Cain acquired after murdering Abel (Gen. 4:17).

The genetic problems resulting from familial intermarriage would not have existed at that time since the earth and human biology, in their original state at the beginning of time, was “very good” (Gen. 1:31).  Additionally, the laws prohibiting incestual marriages would not come from God until centuries later at Sinai (Lev. 18:7ff; 20:11-21).