I remember reading a book when I was around 10 that was about an apocalypse and only two teenagers survived it. I think they were brother and sister but unsure. At some point they were discussing that they should have children and that they would also need to have children with their children to ensure survival of the human race. It was really weird and my parents wouldn’t have let me read it if they knew about that. They also had it moved from the kids section of the library.