First, "Therefore, even as through one man sin entered into the world, and death by sin, and so death passed on all men inasmuch as all sinned." [Romans 5:12]
We read in [1 Corinthians 15:45] that Adam was "the first man." God did not start by making a whole group of men.
Second, in [Genesis 3:20] we read, "And Adam called his wife's name Eve; because she was the mother of all living." In other words, all people are descendants of Adam and Eve -- she was the first woman.
Third, Cain was the first child of Adam and Eve recorded in Scripture [Genesis 4:1]. His brothers, Abel [Genesis 4:2] and Seth [Genesis 4:25], were part of the first generation of children ever born on this earth.
Fourth, back at the beginning, when there was only the first generation, brothers would have had to have married sisters or there would be no more generations.
We are not told when Cain married or any of the details of other marriages and children, but we can say for certain that SOME brothers had to marry their sisters at the beginning of human history.
Fifth, many people immediately reject the conclusion that Adam and Eve's sons and daughters married each other by appealing to the law against brother-sister intermarriage.
The law forbidding marriage between close relatives was not given until the time of Moses [Leviticus 18-20]. Provided marriage was one man to one woman for life [based on Genesis 1 and 2], there was no disobedience to God's law originally when close relatives (even brothers and sisters) married each other.
Finally, today, brothers and sisters (and half-brothers and half-sisters, etc.) are not permitted by law to marry because their children have an
unacceptably high risk of being deformed.
Adam and Eve did not have accumulated genetic mistakes. When the first two people were created, they were physically perfect. Everything God made was "very good" [Genesis 1:31], so their genes were perfect -- no mistakes! But, when sin entered the world (because of Adam -- [Genesis 3:6], [Romans 5:12]), God cursed the world so that the perfect creation then began to degenerate, that is, suffer death and decay [Romans 8:22]. Over thousands of years, this degeneration has produced all sorts of genetic mistakes in living things.
Cain was in the first generation of children ever born. He (as well as his brothers and sisters) would have received virtually no imperfect genes from Adam or Eve, since the effects of sin and the Curse would have been minimal to start with (it takes time for these copying errors to accumulate). In that situation, brother and sister could have married with God's approval, without any potential to produce deformed offspring.
By the time of Moses (a few thousand years later), degenerative mistakes would have built up in the human race to such an extent that it was necessary for God to forbid brother-sister (and close relative) marriage [Leviticus 18-20]. (Also, there were plenty of people on the earth by then, and there was no reason for close relations to marry.)