This is a very highly controversial topic among the scholars. The two opinions are as follows:
1. It was allowed then siding the tone of Adam did brothers and sisters to marry. I.e. Eve would always give birth to sets of twins each twins were a boy and a girl. So that the boy would marry the girl from the second set of twins and not from his own set of twins.
2. Second opinion which is the more reliable one is that initially until Adam and Eve were are able to procreate Allah created for them beings to marry that were not their children and after they grew in number humanity also grew through marriage.