Wanna correct on "Fusre Jogi": Adam and Eve weren't created as brother and sisters. It is a wrong interepretation. Read the book of Genesis, if you can. Eve was formed later from Adam's body part that is one of Adam's ribs and she was given to him  in the same respect so he (Adam) can find his soulmate in her (Gen. 3:20). As far as physical relationship of their offspring: that was a special circumstance that had to happen, because there was no other choices to continue the generation of mankind. God let it happened because of that. On the other hand, God had already kicked Adam and Eve out of Garden of Eden because of thier sin (disobey). So that makes perfect sense why Adam's offsprings messed each other.