Nobody remembers Seth. And, Cain killed Abel.
But, Adam and Even had more children, both sons and daughters. So, Cain married, and had children with his sister. So did Seth.
The only recorded story in the Bible of a parent having a child with his child is Lot with his two daughters, but that was so much later, and it was the older daughter's idea, because at the time, it was the culture of women around the world to continue their blood line, and other women would look down on women who did not have children.
The people during Cain's day had no choice but to marry their sister, because they were the only one's available to marry. Humans were just starting to populate for the first time ever, in the history of the world. Once humans were populated enough, by the time of Moses, there were enough people to where we no longer needed to marry our sister, so the law of Moses forbade incest. So, from that time on, incest has been taboo, and genetic deformities, due to incest, came to be.