incest doesn't really make sense in a post-apocalypse setting. no government, no laws, no coherent society to frown on it. in that world nobody bats an eye if dad fucks everyone in his family. nobody cares.to do something like this in a post-apocalyptic world
even in GOT cersei and jamie were outlaws to their own system and risked losing everything for doing it, and later cercei was thoroughly shamed and punished for it. you need some kind of systemic indignation or relatives have no specific reason to not fuck if they happen to like to.
in a post-apoalypse world incest is just tuesday.