These days I understand why the law exists, but why was it commanded from the beginning? It's still a little bit difficult to digest, but it's obvious that in Genesis siblings had to have relations to multiply. I also know that there's always an acception to the rule, but why did God command this from the beginning so? Couldn't it have been a law given much later on when it was possible?
Perhaps it is to convey that the laws of forbidden unions are essential to man from his very beginning and that the exception was for pragmatic reasons only. The concept of the prohibition, however, still existed. (Had the prohibition started later, that might have suggested that the law was not as fundamental as the other laws and was limited to a specific period of time.)