Adam and eve had many children,not just cain and able.Cain took for himself one of his sisters as his wife.Remember that adam and eve were both perfect in mind and body(part of the reason adam lived so long)and would have passed on these near perfect genes to their children.Hence cain and his sister could have had children with out them being backward or mentally retarded.Also Gods original plan was for man to populate the earth so it could not have been considered incest as it was gods will.We are so far removed now from physical perfection thats perhaps why incest is frowned upon?Hope this helps.
