Adam and Eve's children would have married each other. At the beginning of time, though, this practice was acceptable in the eyes of God for the obvious reason that the human race could not have continued without it. But we should not see this as a contradiction to God's moral law, i.e., that He makes certain exceptions merely for the sake of convenience. We know today that inbreeding can be a dangerous health hazard. When close blood relatives marry (who share a common gene pool), the chance of passing on a lethal gene combination to their children is greatly increased. The average human being today carries four to six lethal genes. They can be caused by deletions and mutations of genes in the gene structure of the cell, and these lethal genes are what caused problems with inbreeding. However, since Adam and Eve were God's original couple, they would have had the purest genetic code possible. (My answer continues in the comments)
Abel died. There are none of either's children alive today. It is believed by many that Abel never had children, and that his brother Cain had a son named Enoch (Genesis 4:17). Cain's descendants all died at the flood, ending his lineage, so that after the flood, the only surviving humans were (and are) descendants of Seth. (1 Chronicles 1:1-4)
In later writings, someone came up with more children, including females, who then commited incest. Incest was as big in the bible, as the number 40.I don't know if the story about the added children was ever added to the original text. But it was accepted by many. there is a second story of Genesis, called the book of Enoch, pieces of that manuscript are found in the Dead Sea scrolls..
Adam and Eve had many other children, daughters and sons. Its also possible that God had placed other people on earth by then. He didnt tell us He did but that doesnt mean He didnt. There were the Nephelim for one which arent really explained other to call them 'sons of God' and great warriors.
Adam and Eve had hundreds of sons and daughters, they lived over 900 years. The gene pool at creation was perfect, therefore brother and sister married. By the modern age the gene pool has become so contaminated the closest relatives to marry is third cousin.
Adam and Eve had more children besides Cain and Abel. Obviously these children and/or cousins married and had children, and the earth was populated. At that time, humans still had outstanding physical health and vitality, and lived long lives. The likelihood of passing on defects as a result of marrying a close relative was not great.