http://www.gotquestions.org/Cains-wife.html
That's a biased, 'apologetic' religious site, full of faith-based speculations, (as can be shown below):
"The Bible does not specifically say who Cain’s wife was. The only possible answer is that Cain's wife was his sister or niece or great-niece, etc."
In other words, a concession that family members had interbreed precedes speculative claims to 'excuse' such interbreeding.
"The fact that Cain was scared for his own life after he killed Abel indicates that there were likely many other children and perhaps even grandchildren of Adam and Eve already living at that time. Cain's wife was a daughter or granddaughter of Adam and Eve. Since Adam and Eve were the first (and only) human beings, their children would have no other choice than to intermarry."
So, the point about interbreeding is not being disputed. Moving on to the 'excuses' then ...
"God did not forbid inter-family marriage until much later when there were enough people to make intermarriage unnecessary."
"Unnecessary"? At this point, speculative and irrational 'rationalizations' come into play.
"The reason that incest today often results in genetic abnormalities is that when two people of similar genetics (i.e., a brother and sister) have children together, there is a high risk of their recessive characteristics becoming dominant."
Just so and because precisely that inter-marriage/inbreeding was conceded, such recessive genetic characteristics would soon arise in the 'extended family'.
"When people from different families have children, it is highly unlikely that both parents will carry the same recessive traits."
The population described did not consist of "people from different families" however. It alledgedly arose from the _same_ family, (whose gentic codes would have carried both recessive and dominate genes).
"The human genetic code has become increasingly “polluted” over the centuries as genetic defects are multiplied, amplified, and passed down from generation to generation."
That would be as a result of initial inbreeding, combined with genetic mutations caused by environmental radiation.
"Adam and Eve did not have any genetic defects, and that enabled them and the first few generations of their descendants to have a far greater quality of health than we do now."
Such a claim is entirely speculative and is a 'faith-based' one which presumes 'perfect creation' within the theory of 'creationism'. There is no evidence to support such creationist claims.
"Adam and Eve’s children had few, if any, genetic defects. As a result, it was safe for them to intermarry."
You can have it both ways though. Either the interbred offspring had no genetic defects, (and therefore, passed none on to the extended family they were breeding), or such later genetic "pollution" occurred as a result of such inbreeding, (combined with background radiation?). This means that any resulting genetic defects observed later took awhile to be attributed to breeding with immediate family members, (a practice which would have ceased without any prohibitions from hypothetical supernatural decrees due entirely to 'trial and error').