How did Adam and Eve's children have children?

Incest...it would've been the only option.

Assuming you believe that story actually happened.

So the story goes every time Eve conceived she had twins and once they grew older she would not allow the same twins to marry each other but to the children of the other barring.

Somebody once pointed out that "incest" was a matter of taking the idea of brotherly love one step too far. In this case, since Eve was the only woman on the planet, I'd say the three men in her life would have kept her as busy as a one-armed paper hanger.

From what I've read Cains line isn't in the genealogy so some believe his father was "a little different."

Also in some texts as Thrice says, they each had a twin sister. In those Cain killed Abel because he wanted to marry his own twin not Abel's.

In the bible it talks of a "holy seed" which suggests a non holy seed ie there were more than one lineage.

There are also two Genesis, the first is where God made man and woman at the same time.

Finally you have "The sons of God" marrying the "daughters of men" hence another lineage, for this reason God sent a flood.

The Bible states that Adam " became father to sons and daughters. (Genesis 5:4)  So Cain took one of his sisters or perhaps a nieces as a wife. (Genesis 4:17) Later, God's Law to the Israelites did not permit the marriage of a fleshly brother and sister. Leviticus 18:9

If you believe that story, isn't the answer quite clearly incest?

I suspect the point of Genesis is that all "humans" are directly connected to the first human being and share a common humanity.

I also suspect that no one at the time would have appreciated an explanation based on "a diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum."

If the Bible is taken Literally, it would be the act of Sisters and Brothers having sex to procreate . . . At which point several birth defects would result and continue to degrade the human conditions until God could drown the majority of mentally impaired off springs to start over again with ONE family just off the boat . . .

However, if taken allegorically . . . It is still a heinous myth that many base their lives off of . . .

It definitely was not incest! Circumstances were different in earlier times. Adam and Eve were created perfect, and the instruction for them ‘to multiply and fill the earth’ would necessarily require that their offspring marry one another and reproduce. (Gen. 1:28) But as perfect humans, their children would have been perfect as were their parents. Even though Adam and Eve sinned and became imperfect, Cain and his brothers and sisters were still so near to physical perfection that the children they produced did not suffer the same adverse effects as do children born of such unions today. Even some 2,000 years afterward, God’s faithful servant Abraham married his half-sister Sarah, and God did not disapprove. (Gen. 20:12) It was yet another 450 years or so before God saw fit to provide his nation of Israel a body of laws that forbade incest on penalty of death. (Lev. 18:8-17) By that time imperfection had apparently developed to such an extent that no longer was it safe for close relatives to marry.