Cain and Able weren’t Adam and Eves only children. Keep in mind that canonically, Adam lived for over 900 years. That’s a lot of time you can use to make more babies.
After the whole regicide incident, they also had Seth. Both him and his exiled brother ended up marrying their sisters. Their descendants did the same. So yeah, it’s all incest, top to bottom.
The biblical flood was actually created to wipe out the descendants of Cain, since Noah and his cousins wife were both descendants of Seth
So what you’re saying is I should drink the water that drips out of my exhaust when I start the car on colder days?