The Bible never says that Cain and Abel were the first two children of Adam and Eve. By the time they were born, there may have already been many brothers and sisters. The reason the Bible focuses on them is because of what happened between them. The primary purpose of the Bible is not to give all the historical details, but to teach spiritual lessons. So clearly, Cain's wife was a sister or cousin. The practice of marrying family members was not outlawed by God until the law of Moses. According to Jewish tradition, Eve gave birth to 33 sons and 22 daughters. If each daughter began to have babies around the age of 20, it wouldn't take long for the earth to have quite a large population.