Seems many are confused with the question, but if you understand, this one thing, it becomes simple.
Adam and Eve were eternal at one time, and had thousands of children. Adam and Eve had been blessed to be fruitful and multiply which is exactly what had been going on
Gen 1:28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it.......
Cain and Able were only mentioned after Adam and Eve were banished from the garden and lost eternal life. Cain and Able were highlighted because of the murder that took place.
The people in the land of Nod were all offspring of Adam and Eve
If you notice the age Adam lived to, then it was possible to populate the earth in huge numbers.
So was it incest before the sin?? no because its the same with all earth creatures it was the only way creatures could multiply in the pure form before the sin entered the world, hence animals also carried a blessing prior to the fall of man
Gen 1:22 And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth.
Its only incest after the fall of man.