"who says the sciptures disallows such? I gave you the example of what's written in the scripture that "promotes" incest, why would you tell me that it's a sin as written in the Bible?"
Well Coleacanth I am not talking about Bible because I don't know much about what is in it. I talked about Quran which clearly prohibits incest.
You know alcohol was also allowed for muslims before it was made forbidden. Likewise the scriptures do say God created only a pair of male and female that grew the human race. Hence there was no way other than meeting two siblings to further human race. As the generation grew the same was made forbidden for believers.
Now don't ask me why your God didn't create multiple couples than just Adam and Eve. We muslims don't question God's wisdom....