Intermarrying among Adam and Eve's children

Praise be to Allah.

All Praise be to Allaah.

As long as you are confident in your heart about your belief, no such insinuations will hurt you (inshaa' Allaah). If a doubt strikes a believer or something appears to him as a conflict between different textual sources of Shari'ah, he must believe that indeed for such doubts there are true answers and for such apparent conflicts there are solutions even if he doesnt know it or is unable to find the right answer himself. However, a person should not collect all the doubts in his heart and worry about the complexities and leave aside the learning of useful knowledge. This is because it is incumbent upon a Muslim to learn and know things which have been clearly described (Muhkamaat) in Shari'ah so that he may be able to repel the doubts.

Regarding the question you have asked, it is a known fact that legislation differs from one Shari'ah to another, while the principles and beliefs remain the same in all of them. So, making of portraits was allowed in the Shari'ah of Sulayman (peace be upon him) but is prohibited in our Shari'ah. Similarly, making prostration of salutation was permitted in the Shari'ah of Yusuf (peace be upon him) but is illegal in ours. Also, war booty was prohibited for nations before us but it is completely legal for us. The Qiblah of people before us used to be towards Bayt Al-Maqdis, but for us it is towards Ka'bah. In a similar way, marriage between brothers and sisters was permitted in the Shari'ah of Adam (peace be upon him) as opposed to those that came afterwards. The following is a clarification on the issue by Haafidh Ibn Katheer, who said:

Allaah allowed Adam (peace be upon him) to marry his daughters to his sons for necessity. Every couple used to have a boy and a girl. Hence, he married the girl of one couple to the boy of another. This is said by Suddi regarding what has been narrated by Abu Maalik and Abu Salih, from Ibn Abbas, by Murrah from Ibn Masood and by other companions of the Prophet (peace be upon him) that Adam did not have (in his grandchildren) a baby boy unless it was accompanied by a girl, so he married the male of a couple to the female of another, and the female of a couple to the male of another.