Please can you tell me how world population grew when Adam and Eve were a single couple and their children brothers and sisters. What does the Qur'an and religious literature have to say on this issue? Did the children marry one another then? Is this not weird?
The basic sources of divine origin don't provide any information about the matter. The Holy Qur'an, reliable sayings of the Prophet (sws) and the Bible do not give any detail in this matter.
Obviously, the only possibility in this regard is the one you have pointed out. The progeny of Adam and Eve the first pair of human beings must have married one another.
As far as the legitimacy of marriage between siblings is concerned, it needs to be appreciated that the basic reason for prohibiting marriage with blood relations is to sanctify and protect the institution of family. Before the development and establishment of this institution the restriction of course could not have been enforced. It was only after this institution developed that the restriction of marriage between siblings seems to have been enforced.