At some point/s in human history, we mixed our DNA with that of the Neanderthals (lots of that) and the Denisovans. There were a lot of other hominids and hominins around, and we may have had interbreeding with them too. The level of intelligence of such folks is unknown. Did they already count as descendants of Adam and Eve, or not? (I would say yes, because the Neanderthals buried their dead with religious ceremonies, and there is evidence that one of the early hominin groups did also.)
Basically, we don’t know if this stuff was happening before Adam and Eve, or after Adam and Eve. We don’t know if it was blameless behavior, or sinful behavior. We just don’t know enough about the history of prehistory, and we probably never will.
OTOH, if you want to read Bible fanfic and oral traditions, there are Jewish texts that will tell you all kinds of stories about Cain and Abel’s sisters.