So god created Adam and Eve. Then for human race to continue, who would have Adam & Eve's children procreate with? Among themselves?
There is another theory about it.
Adam & Eve's children did not marry with each other. but from Junnah "Hoor" were sent down. for each male(boy) there was one "hoor". Habeel and Qabeel disputed on the same issue. The "hoor" which was sent for Qabeel, he did not like it. then Allah asked for scarifice. and so on.
The same people who believe in it says it is haram to marry between brother and sister since the days of Adam &b Eve. (AS).