I mean, in the KJV at least there's adam and eve, then cain and abel. then just cain, and then the book just says "Cain went to the land of Nod, east of Eden. And Cain knew his wife." So there were definitely other people around and canonically we're not inbred, it's just that we all have a common inheritance from Cain, and therefore Eve, and therefore we all inherited original sin.

It makes a lot more sense when you remember that YHWH was originally a bronze age thunder and warrior god who was not the one and only god like it says in christian canon. He was a god amongst many gods, and only ever asserted himself as the one and only god of the tribes of Judea (who were, at the time, polytheistic). It's the difference between your girlfriend saying you're not allowed to date anyone else and your girlfriend saying she's literally the only girl that has ever existed and will ever exist. It's not until after the Jews lost their war with the Babylonians and Solomon's temple was destroyed that we start seeing YHWH spoken of as the creator and the only God. Early versions of the creation story even have YHWH using the first person plural when making declarations, implying that the audience was other gods which would have been easily accepted at the time in which the stories emerged but are absolute anathema to Christians and Jews today. From memory, after Adam and Eve eat the apple YHWH says something like "They have eaten of the tree of knowledge, and now know of good and evil. If they eat from the tree of life they will become like us."