How could Adam and Eve’s family populate the world without committing incest?
To violate a law (incest), you have to first have a law.
In Adam’s day, humans lived by their conscience. God had not given laws yet. Additionally, there was no medical reason at that point that would prohibited close relations from marrying and reproducing. The degeneration of the human gene pool had only just begun shortly before with Adam’s first sin and the resulting curse.
Only the first generation would have needed to inter-marry. After that it would have been cousins, and successive generations would widen the family gap. Still, until God gave the Law, there was no prohibition forbidding interj-family marriage, and for many generations, no medical risks.
Incest only became a sin, once God pronounced it a sin later on after the world was populated. Because of the innate sense of discomfort about reproducing with a family member, its doubtful that is was a common practice anyway (my opinion).
Either way, there would have been no medical consequence like there is today with retardation a probability in incest due to genetic degradation.
We make the mistake of thinking that everything in history has always been the same as it is now. When sin entered the human race, it began a corruption and degradation that continues to intensify with every generation.
This question is often proposed as a “trick question” meant to “stump” a Christian and make the Bible seem flawed. But like all “trick questions” concerning the Bible, there is no trick at all.
Adam and Eve’s children could intermarry and reproduce without breaking any law or commandment, and without fear of genetic problems.