Q: How could there be be descendants from Adam and Eve, if there were only 2 sons mentioned, and if there were sisters, how could they multiply within the same family?
A: Thanks for this question to us here at TruthBook.com. It is a multi-facted question, so please bear with me while I try to sort it out for you...the answer to your question is not all that simple. You don't mention whether you are a Urantia Book student, but since you have asked the question as you did, I am going to assume that you are not all that familiar with the story of Adam and Eve as revealed in its pages...I will offer quotes from The Urantia Book, and provide you with links that you can follow so that you'll be able to learn all about it at your leisure, if you so choose.
Adam and Eve, as presented in the book of Genesis, were supposedly the first two human beings. The Biblical account tells us that they had two children—Cain and Abel—and then ate from the forbidden fruit and were banished from Eden. However, The Urantia Book reveals the true identity of Adam and Eve, and the details of their true purpose and mission to our planet.
Far from the simplistic fable from the Bible, the story of Adam and Eve is a complicated one, and not well-understood by most of the world as yet. In The Urantia Book, we are given a complete picture of this amazing series of events—the mission of Adam and Eve was actually the third of five epochal revelations to our planet...
The human descendents of Adam and Eve sprang partially from the two children that were conceived as a result of default—Cain and Sansa.
Cain was, in reality, the son of Eve and Cano, and Sansa was the daughter of Adam and Laotta. Both Cano and Laotta were human beings—not members of the Violet race to which Adam and Eve belonged. If you have read this story (in the link above), you may recall that the liaison of Eve with Cano meant the default of the Adamic mission. When Adam realized what Eve had done, he "sought out Laotta, the brilliant Nodite woman who was head of the western schools of the Garden, and with premeditation committed the folly of Eve." And these were the acts which effectively defaulted their mission and doomed both Adam and Eve to their degradation to mortal status.
If all had gone according to the divine plan, Adam and Eve would have procreated between themselves, and then their children would have procreated with each other (this was safe for them, and standard practice, as they were a pure-line race) until their numbers had reached one million. Following this milestone, the pure-line children of Adam and Eve would have begun procreating with the higher races of humanity then living on the earth. In this way the Material Sons and Daughters upstep the biological systems of the races, both physically and spiritually, through their life plasm. And this is the mission of an Adam and Eve to an evolutionary planet such as ours. Theirs is a mission of physical, intellectual, and spiritual upliftment to the evolutionary races of mankind. They are sent from celestial government as a revelation from God, and as biologic uplifters.
Please see Racial Amalgamation—Bestowal of the Adamic Blood. This section explains the normal course of events when a Material Son and Daughter (Adam and Eve) are bestowed on an evolutionary planet, and how Adam and Eve's genetic patterns are distributed throughout humanity.
The two sons of Adam and Eve that most people know about from the Bible are Cain and Abel. However, in The Urantia Book, we learn that Cain, as mentioned above, was the offspring of Eve and Cano, but Abel was the offspring of Adam and Eve—the first child of theirs to be born after the default, and the move to the second Garden. Before this move, Adam and Eve had produced 1647 children, and another 42 were produced after the default, besides Cain and Sansa. In the ensuing years, Abel was killed by Cain.
You can read more about Cain and Abel HERE
And so, these two children of the Material Son and Daughter—Cain and Sansa—were the first mortals to be produced as a result of mixing Violet blood with human races. Again, this was not supposed to take place until the children of Adam and Eve had reached one million. and it was never to have happened by Adam and Eve themselves...but Sansa and Cain both produced children, and the racial mixing proceeded.
From The Urantia Book:
And here is how the Violet race was deliberately added into the human races of those times, after the default of the original plan:
76:0.2 Cain and Sansa were both born before the Adamic caravan had reached its destination between the rivers in Mesopotamia. Laotta, the mother of Sansa, perished at the birth of her daughter; Eve suffered much but survived, owing to superior strength. Eve took Sansa, the child of Laotta, to her bosom, and she was reared along with Cain. Sansa grew up to be a woman of great ability. She became the wife of Sargan, the chief of the northern blue races, and contributed to the advancement of the blue men of those times.
76:2.8 Cain’s life in Mesopotamia had not been exactly happy since he was in such a peculiar way symbolic of the default. It was not that his associates were unkind to him, but he had not been unaware of their subconscious resentment of his presence. But Cain knew that, since he bore no tribal mark, he would be killed by the first neighboring tribesmen who might chance to meet him. Fear, and some remorse, led him to repent. Cain had never been indwelt by an Adjuster, had always been defiant of the family discipline and disdainful of his father’s religion. But he now went to Eve, his mother, and asked for spiritual help and guidance, and when he honestly sought divine assistance, an Adjuster indwelt him. And this Adjuster, dwelling within and looking out, gave Cain a distinct advantage of superiority which classed him with the greatly feared tribe of Adam.
And so Cain departed for the land of Nod, east of the second Eden. He became a great leader among one group of his father’s people and did, to a certain degree, fulfill the predictions of Serapatatia, for he did promote peace between this division of the Nodites and the Adamites throughout his lifetime. Cain married Remona, his distant cousin, and their first son, Enoch, became the head of the Elamite Nodites. And for hundreds of years the Elamites and the Adamites continued to be at peace.
And so, even though the mission of Adam and Eve was not completed as planned, the human races of Urantia have still benefited from this infusion of Adamic blood.
76:4.8 After becoming established in the second garden on the Euphrates, Adam elected to leave behind as much of his life plasm as possible to benefit the world after his death. Accordingly, Eve was made the head of a commission of twelve on race improvement, and before Adam died this commission had selected 1, 682 of the highest type of women on Urantia, and these women were impregnated with the Adamic life plasm. Their children all grew up to maturity except 112, so that the world, in this way, was benefited by the addition of 1, 570 superior men and women. Though these candidate mothers were selected from all the surrounding tribes and represented most of the races on earth, the majority were chosen from the highest strains of the Nodites, and they constituted the early beginnings of the mighty Andite race. These children were born and reared in the tribal surroundings of their respective mothers.
Multiplying within the family of Adam and Eve was accepted practice, and is the way things are done on evolutionary planets such as ours; however, since our planet had been laboring under the effects of the Lucifer rebellion things did not go as planned. Thus, we have this very irregular turn of events.
This has been a long reply, and I hope not too difficult for you to take in...I hope that this reply answers your questions about how Adam and Eve left descendents on our planet.
If this is the first time that you're hearing this story, I hope that you'll please have a look at the links I have placed into this reply, so that you can gain a new understanding of this most amazing pair, and this most amazing story in our distant past—a story that is still playing out on our world today.
The Urantia Book recital of the Adam and Eve fable places this actual event in historic perspective. Human beings had existed nearly one million years before Adam and Eve arrived here. They were to establish the most advanced culture on the world and then begin the process of uplifting all the native peoples of the world through improved genetics, extensive education, and valuable spiritual teaching. They failed in these regards but much good resulted from their failed attempt. A fable about Adam and Eve was written more than 35, 000 years after they lived and died here and this incomplete Bible story is all we'd know about this exceptional couple without the instruction of The Urantia Book.
Thanks so much for writing, and please write back if you have further questions...