Adam and Eve (Hawwa)' s children got married. But there were no any other population so whom got married with their children? How did the children of Adam marry each other?

A Brief Description of the Question: 

Adam and Eve (Hawwa)' s children got married. But there were no any other population so whom got married with their children? How did the children of Adam marry each other?

The Answer: 

Humans grew in number by being born from Adam and Eve. Our mother Eve was having twins every time she was giving birth. One of them was always a boy and the other was a girl. Adam was making the twins who were born at the same time marry the previous or the next twins. The girl who was born with Abel was ugly but the girl who was born with Cain was beautiful. Under those circumstances, Adam wanted Abel to marry the girl born with Cain and Cain to marry the girl born with Abel. However, Cain did not consent that, he did not want to give the beautiful girl born with himself to Abel and he wanted to marry with her himself.
Adam did not allow that and he handed over the matter to Allah. According to the order coming from Almighty Allah, he told both of them to present a sacrifice to Allah and he said the sister of Cain would belong to the one whose sacrifice was accepted. Thereupon, Cain offered a bunch of wheat and Abel presented a sheep as a sacrifice. A fire coming down from the sky took Abel’s sacrifice but Cain’s sacrifice remained on the earth. In that situation, Abel was justified and he deserved the girl. However, Cain lost his temper completely. That event is told in the Qur’an as follows: 

“Recite to them the truth of the story of the two sons of Adam. Behold they each presented a sacrifice (to Allah): it was accepted from one, but not from the other. Said the latter: be sure I will slay you. Surely, said the former, Allah does accept of the sacrifice of those who are righteous.”

“Even if thou stretch out your hand against me to kill me, I shall not stretch out my hand against you to kill you; For, I fear Allah, the Lord of the Worlds. I would rather you should bear the punishment of the sin against me and your own sin and become one of the owners of the Fire. That is the reward of evil-doers.”

“The (selfish) soul of the other led him to the murder of his brother: he murdered him, and became (himself) one of the losers. Then Allah sent a raven, who scratched the ground, to show him how to hide the shame of his brother. Woe is me said he; was I not even able to be as this raven, and to hide the shame of my brother? Then he became full of regrets.” Qur’an, The TableSpread (Al-Maidah); 27-31 (5: 27-31)

To get to the position of the marrying of the children of Adam with each other; from Hazrat Adam to the Prophet Muhammad (pbuh), all prophets have taught the righteous religion. The fundamentals of the faith which are the basis of the religion have always remained the same. However, in the Shari’a ( Divine laws), which are the deeds related to the ibadah (worship) and the world, some ordinances have come by changing according to the requirements of every age from Hazrat Adam to the Prophet Muhammad (pbuh) and the needs of the people. Almighty Allah has set a different Shari’a by taking the way of living and the benefits of every age into consideration. In the 48th ayah (verse) of the Surah al-Maidah, “To each among you have we prescribed a law and an open way.” Was stated about the matter.

Badiuzzaman explains that matter as follows: “Sacred laws change according to the ages. Indeed, in one age different prophets may come, and they have come. Since subsequent to the Seal of the Prophets, his Greater Shari‘a is sufficient for all peoples in every age, no need has remained for different laws.”  (Said Nursi, Risaleinur Collection, the Words, Twenty Seventh Word)

For example, although the Jews can only worship in synagogues and the Christians in churches, we Muslims can perform the salats (prayer) everywhere. Still, while the suet of the animals such as cows and sheep is haram (forbidden) in the shari’a of Prophet Moses, it is halal (permissible) in our religion.

Adam is the first human and the first prophet. Allah also has sent and taught a religion and a shari’a to him, too. He used to act in the way Allah has showed him. Almighty Allah permitted marrying of Adam’s children with each other because of an exigency. Because, the progeny of the human beings had to increase. Since there were no other humans, the sisters and the brothers had to marry each other as an exigency. That practice continued for some time; but when the number of the humans increased, there were no more need and exigency to such marriages and that practice was abolished.

As Allah has created Eve from the rib bone of Adam as a partner to him, He can also create the sisters and the brothers born in different times as strangers to each other. Afterwards, the human kind increased and He prohibited the marrying of sisters and brothers even if they were from different twins.

Being halal of that marriage is based on the command of Allah. Because, the badness of something is because of Allah’s prohibiting it and the favorableness of something is because of Allah’s commanding it or unrestricting it. That is, what Allah commands becomes favorable and what Allah forbids becomes bad.