Why was it not a problem for early Bible people to marry their siblings?
Early siblings could marry because the human race was still genetically pure, so close unions produced no defects. Brothers married sisters for the first generation or two without problem. Even Abraham, about 2,000 years after Adam, married a half-sister, and Isaac was born without defect; the taboo came later as the genetic pool weakened.
Les explains that since only Adam and Eve were created, Cain's wife must have been a sister or close relative. For the first generation or two, brothers married sisters, and that was no problem because the human race was so pure genetically. Even as far along as Abraham, 2,000 years after Adam, Abraham with God's blessing still married a half-sister, and there was nothing wrong with Isaac. God did not put a taboo on marrying close relatives until sometime even after Abraham, because by then the genetic pool was being weakened and the known problems began to arise.