How did the children of Adam and Eve intermarry despite the prohibition of brothers marrying sisters in Islam?

A Muslim should not make his heart a repository for doubts, but rather, he should know the clear-cut matters in Sharia. Rulings differ with the differences in divine laws, while the fundamentals and beliefs remain constant. Examples include the permissibility of statues, prostration of greeting, and war spoils in previous laws, which are forbidden in our law; and the direction of prayer for previous nations towards Jerusalem, while ours is towards the Kaaba. Adam was commanded to marry his daughters to his sons due to the necessity of the situation.