Because of the effects of sin, mistakes have added up in the genes of humans so that today, deformities could result from close relatives marrying. Adam and Eve's children would have had relatively few mistakes in the genes compared to today. So, provided it was one man for one woman for life - there would not have been a problem at all for Cain to marry his sister!

Did Adam have a Bellybutton?: And other tough questions about the Bible (2000)