Yes there is a big difference between the 2, but look at how big and rife internet exploitation of children is becoming.Whos to say it won't ever happen?God forbid it ever does!
God told Adam and Eve to "fill the earth" which they did,so yes it is considered that their children would have married each other.The human race was genetically pure then so there was no guilt of inter-marriage.
Now if you believe that Adam and Eve started the human race then we are all related from this bloodline.It was later on that God decreed that immediate family members should not marry.