People with a "Where did Cain get his wife from?" kind of mindset who like to try to disprove God's word point out that if Eve was the mother of all living, then in order for humankind to continue, one of her sons must have married his sister, which was a sin.That's where their logic comes unstuck. It wasn't a sin.
When Adam and Eve were created, their genes were perfect. The genes of their children would be near perfect.
By Moses' time, hundreds of years later, some defective genes would have got into the gene pool. It was then that God decreed that a person was not to marry a near relative.
If I were to marry someone outside my own family, their genes would be different to mine. A defective gene in me would be likely to be overcome by a good gene at the same point in my spouse, so that our children would not be affected.
If I married a close relative, our genes would be similar. A defective gene in me would be likely to be matched by a defective gene at the same point in my partner, so that our children would be genetically defective.
The more you understand about creation, the more you see the wisdom of God.
