When God created Adam, He created him perfect. He then created Eve by taking a part of Adam out of him, and making Eve from it. They were therefore in effect truly one flesh. This was obviously not a problem to God, as He could have created two or three or more families in the beginning. Instead He told them to multiply and fill the earth. And from these two perfect humans came all of the humanity on the earth.

Genesis 1: 27 So God created man in His own image; in the image of God He created him; male and female He created them. 28 Then God blessed them, and God said to them, “Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth.

Acts 17: 26 And He has made from one blood every nation of men to dwell on all the face of the earth, and has determined their preappointed times and the boundaries of their dwellings,

After the flood the situation was slightly different, as there were now four pairs of people to repopulate the earth. God gave Noah and his sons the same instruction to fill the earth as He gave to Adam and Eve.

Genesis 9: 1 So God blessed Noah and his sons, and said to them: “Be fruitful and multiply, and fill the earth.”

We have to remember that with perfect humans there was very little degeneration due to sin in the beginning, which is why people lived for up to almost a thousand years in age, before the flood. So marrying one’s sister did not have the same problems attached to it that we have now, after six thousand years of degeneration. At present incest makes genetic problems so much more of a possibility, which was not a problem with the children of Adam and Eve.

So, to answer your question, yes, Adam and Eve’s children married their own brothers and sisters, as there were no other humans around. But this was according to God’s plan, and caused no genetic problems. Even after the flood, Abraham married Sarah, who was his half-sister, and Isaac married Rebecca, who was his cousin. Only much later did God have to stipulate that close family were not allowed to marry, due to the genetic problems it could cause. I hope this is satisfactory and does not shock you. God bless!