No need for incest.

Eve was Adam's second wife.

His first one was created at the same time as he was. Genesis 1:27

He didn't 'consumate his marriage' with his first one, Lilith, because she left him and found herself a nice demon and had sex with him.

Adam gave 'the woman' the name Eve (Genesis 3:20) because he had been told to name all the animals. That was after they had been thrown out of the garden of Eden and finally had sex. Genesis 4:1

Have a look here at the legends that sprang up later: [ewao.net]

So Cain and Abel could have taken wives from among the daughters of Lilith and Samael.

Ha, ha. I asked that question once and got the answer that "it was necessary back then to be fruitful and multiply". Okay, I can understand that; populating the world has to start somewhere. But then I asked about the inconsistency of allowing incest in this case while saying that the Bible should be taken literally and is always right. Lol.

" well, then the blood line was pure."? What does that mean? After that they had sex with farm animals?

The logical conundrums that religous faith makes for itself are countless.