Praise be to Allaah.
Allah, may He be exalted, created Adam the father of mankind (peace be upon him) and he created from him his wife Hawwa', then mankind spread from them as Allah says (interpretation of the meaning):
O mankind! We have created you from a male and a female, and made you into nations and tribes, that you may know one another. Verily, the most honourable of you with Allaah is that (believer) who has At‑Taqwa [i.e. he is one of the Muttaqoon (the pious)]
O mankind! Be dutiful to your Lord, Who created you from a single person (Adam), and from him (Adam) He created his wife [Hawwa (Eve)], and from them both He created many men and women
More than one of the scholars have stated that Allah, may He be exalted, ordained that Adam (peace be upon him) should marry his daughters to his sons, so he would marry the female from one pregnancy to the male from another pregnancy. Allah, may He be exalted, says concerning the two sons of Adam (peace be upon him) (interpretation of the meaning):
And (O Muhammad صلى الله عليه وسلم) recite to them (the Jews) the story of the two sons of Adam (Haabeel and Qaabeel - Abel and Cain) in truth; when each offered a sacrifice (to Allaah), it was accepted from the one but not from the other. The latter said to the former: I will surely, kill you.' The former said: Verily, Allaah accepts only from those who are Al-Muttaqoon (the pious)'
Ibn Katheer (may Allah have mercy on him) said:
It was narrated concerning them, as quoted by more than one of the earlier and later scholars, that Allah prescribed that Adam (peace be upon him) should marry his daughters to his sons as a matter of necessity. But they said: A male and female were born from each pregnancy, so he would marry the female from one pregnancy to the male from another. The sister of Habeel was ugly and the sister of Qabeel was beautiful. Qabeel wanted to keep her for himself and not give her to his brother, but Adam insisted that they should offer sacrifices and whichever brother's sacrifice was accepted by Allah would win her hand. So they offered sacrifices and it was accepted from Habeel but it was not accepted from Qabeel. And their story was told by Allah in His Book. End quote.
Tafseer Ibn Katheer, 3/82
Ibn Abi Haatim narrated: It was forbidden for a woman to marry her twin brother, and he was commanded to give her in marriage to a brother from another pregnancy. And from each pregnancy one boy and one girl were born. Whilst they were like that, a beautiful girl was born to him, and another was born who was ugly. The twin brother of the ugly one said to his brother: Give me your twin sister in marriage and I will give you my twon sister in marriage. He said: I have more right to my twin sister. So they offered sacrifices, and the sacrifice of the one who offered a ram was accepted but the sacrifice of the one who offered crops was not accepted, so he killed him. Ibn Katheer (may Allah have mercy on him) said: Its isnaad is jayyid.
Tafseer Ibn Katheer, 3/83
See also: Tafseer Ibn Jeeri, 10/206; Tafseer al-Baghawi, 3/41; Tafseer al-Thalabi, p. 732; al-Jaami li Ahkaam al-Qur'aan, 6/134; Zaad al-Maseer, 2/332; al-Bidaayah wa'l-Nihaayah, 1/103
As this is proven from the Sahaabah (may Allah be pleased with them) or some of them, especially from Ibn Abbaas who was the most knowledgeable of people in the interpretation of the Qur'aan, and the scholars narrated what he mentioned, this is not regarded as speculation that it is not permissible to accept.
Rather, in the words quoted above, Ibn Katheer indicated that this is a definitive matter, as he said: Allah prescribed that Adam (peace be upon him) should marry his daughters to his sons as a matter of necessity. End quote.
The words as a matter of necessity indicate that it is not possible that things could have happened differently, otherwise how could all of mankind have come about? How could they have reproduced? There is no way that could have happened except by the sons of Adam marrying his daughters.
And Allah knows best.