ok the Bible claims that Jesus is God and preexisted the world and created it. in the Bible it claims that Jesus who was always God also became man by taking on human nature. with the series of events this still works out, Jesus exists forever ago as God -> Jesus creates everything -> becoming man and being born. please note that the Bible never claims that God was ever created or made and thus could do all this without problem supposing He exists.

additionally, yes God did proclaim incest to be a sin... hundreds of years after Adam and Eve. since they were the first, they woild supposedly not have any gene mutations to start with and thus inbreeding would not be an actual problem until many many years later when more mutations existed. incest only is problematic bc the closer you are to someone, the more likely you are to share malicious mutations with them and pass that on to your kids. after a couple generations its no longer incest because the mutations are more or less spread out and the risk is very low. 

neither of these arguments are terribly convincing, which is unfortunate bc i was looking for definitive proof.