In Genesis 5:4 it actually does say that “Adam had other sons and daughters”. However, none of the daughters are ever mentioned by name.
For human race to have grown there would definitely have been marriages between brothers and sisters. While this type of incestuous relationship may seem like a perversion, it was common even among our Bible hero’s. For example, Abraham married his half-sister Sarah. It was only later in human history, and biblical accounts that incestuous relationships were identified as something that was wrong.