But fifteen hundred and change years later, the punitive Great Flood leaves only four married couples alive—three of the men are brothers, and the fourth couple are their parents.
The book doesn't tell us where Cain and Seth's wives come from, but Adam and Eve are the only established source, so it's probably supposed to be a common sense conclusion.
They almost conceive a child, but Pharinet.
Earnshaw randomly brings Heathcliff home one day, some people do theorize that he's his son by another woman, which would make Heathcliff and Catherine half-siblings.
Arguably, the earlier stories in the Bible were never meant literally, anyway.
And she warned her about it the night Drizzt was born.
Oh yeah, and he hates his brother.
It only happens once, but when they get caught by their social worker who is implied to be atracted to the girl , she teases him by saying "Oh, this has been going on for ages and ages".