God created Adam and Eve as the first human beings on earth. The rest, He didn't considered them as special as Adam. In hypothetically, humans are designed to be a lot different from animals. Incest wasn't allowed. To what Gracy have said as I quote:
I will considered this to be morally wrong. Incest was illegal at all times in the human mind. I'd say, it always has been illegal.
God allowed it in the beginning because how else will they procreate? The Law wasn't given yet.
Show me before the Law where it was wrong to marry close relatives?