If you follow the bibles account of creation. The Lord created man and woman, and told them to be fruitful and multiply. When it became time for either Cain or Seth to be fruitful and multiply, where did the women come from.
The account of creation is actually dealing with things on a spiritual level. And, since to most people that isn't a very well defined term, I'll tell you what that specifically means to me in explicit terms that aren't nebulous.
A spiritual being, such as Adam, Eve, Cain, Abel, Seth, etc. were, is also referred to as a body of "flesh and bone". This is defined to be a gathering of people together who have a common objective and they organize themselves into a distinct body. Think of it as a society. For example, the Roman Empire was a flesh and bone body.
So, when you look at Adam as a distinct society of some kind, then you can start to penetrate what the creation narrative is really conveying. When it says Eve was "bone of Adam's bone" and "flesh of Adam's flesh" it simply meant that some individuals were members of both Adam's society and Eve's society. You could call the Boy Scouts of America a body of flesh and bone. You can call the Baptists or the Methodists bodies of flesh and bone, and so on. You can be members of many bodies of flesh and bone at the same time.
So, if you want to be able to see Adam and Eve for who they truly are, you need to identify them as societies that are very closely linked and intertwined. In a nutshell, they are a body of priesthood authority and a church who function at the highest level of spiritual glory God has ordained to be expressed on this planet. In comparison to them, Christianity is like "fishes" and it isn't a coincidence that they refer to themselves as such.
However, these societies get pretty messed up and become fallen and they are barely recognizable, but you could still figure out what is what if you look hard enough. They are here right now since we are at the juncture in the repeating cycle of things that a new creation is being birthed.
Anyway, so the rules for sexual reproduction of flesh and bone bodies works a bit differently than it does in flesh and blood. I'll try and explain it in unambiguous terms so that a non-religious person can grasp it.
Think of human beings as the raw materials from which Adam, Eve, etc. were created. Humans are the flecks of dust from which Adam was formed. The manner in which this took place is an entity of spiritual power (aka information) became organized within the mind of a human in such a way that you could say he had a "bright idea". This idea greatly excited him as he believed it had great potential and he began sharing that idea with others. He participated in discourse. When others liked his idea they got excited too and they shared it and others joined in. All those who rallied together around that idea became a distinct society and they defined "life" as those who were favorable to the idea and supported it and "death" as those who rejected the idea and fought against it. So, according to their terminology, they were the first "living" Man, the inventors and patent holders of a brilliant new idea. I won't go into the details of it, but part of the nature of this "idea" consisted of the ability to embed within themselves genetic encoding that would more or less assure that their posterity would perform given roles and functions in order to implement their "bright idea". Thus, this only pertained to their actual physical posterity, meaning those who were children of the members of the body of the Adam society. The seeds of eternal life were literally their altered genetics that carried the DNA programming they infused into things so that their "idea" could be carried out. Now, in order for this to work, they needed to have things balanced out between "good" and "evil" so there was also a genetic program for the "forbidden seed" as well. This "idea" includes managing both sides of things. So, the whole illusion of adversarial roles is no more of an issue for them than two partners sharing a beautiful dance they both enjoy immensely. But, for all of the other humans oblivious to all of this are more or less just the dust specks that are pretty much helplessly engulfed by this. That which organized and empowered Adam is to humans like humans are to flecks of dust. So, when Eve came into the scene she produced two sons. One of the "good" seed and one of the "evil" seed so that there would be the two lineages established to carry out the "idea". Remember, each lineage carries DNA programming that makes us vehicles to carry out their "idea". While Cain was literally of Adam's seed, he succumbed to hearken unto the Serpent and became married to one of the Serpent's daughters. Thus, Cain is the carrier of the "forbidden" or "evil" seed and Abel is the carrier of the "good" seed. However, because he was murdered, Seth was conceived in order to be his brother's "kinsman redeemer" (see the levirite marriage law) and raised up seed to his deceased brother Abel. Now, bear in mind, these are flesh and bone beings, but the implications are that we aren't necessitating physical flesh and blood individuals participating in incestuous relations. So, Adam and Eve did have other daughters as well, which could be likened to societies like a church (feminine). And, if you think about it, if Eve was bone of Adam's bone and flesh of Adam's flesh, this was very much like asexual reproduction.
Anyway, I realize I probably carried on a bit long here to make a fairly simple point that we aren't talking about incestuous intercourse via physical copulation between siblings, but rather as discourse via spiritual conversation among the masses of humanity that are aware of it just about as much as the carbon elements in our bodies are aware of their function in such.
The Bible forbids incest as a sin punishable by death under Levitical law.
The Bible says that Cain went to the land of Nod, with his wife after he slew his brother.
If we are going to consider that Cain or Seth, married their sister, then all of mankind are biologically related in the worse way, my great (times several thousand) grand daddy had sex with his sister.....gross and very hypocritical of a god that makes the sin punishable by death in a few books later.
This is one of the many questions I have about that being you call god, it seems when someone like me a pagan witch asks questions like this to a Christian when they are trying to convert me, they say I have a problem with god, hey what you call god is your business, I just wanna answer to my question. Hang around their will be more....blessed be!!
I find your questions quite good, keep them coming.
Maybe this is just a game to him?