Q. How could Adam and Eve have grandchildren if there were no women for their sons to marry?
A. After Cain killed Abel, “Cain knew his wife and she conceived and bore Chanoch…” (Gen 4:17). His wife must have been his sister.
The text does not spell this out, probably in order not to endorse what appears to be incest.
Nor is the birth of Cain’s wife recorded in the text, presumably because the story is more concerned with the sons.
According to the Midrash, Cain was jealous of the beauty of Abel’s wife, who was also a sister, and this was one of the causes of his fratricide.