In the old world it would be wrong, insane even, but when the plague hit and wiped out everyone but us, it started making a lot of sense. I don't know why I'm alive, why my free spirit sister is alive, but we're alone and as we walk the empty world. I think I'm falling for her.
The basic story is of a young pair of kids rebuilding their lives with no boundaries, the sister is a free spirit and after a few months of surviving the 2 are starting to develop incestuous feelings for each other.
As time develops they begin their new family and try and rebuild their world, finding new survivors over the years to form a little community. Think Fallout or the walking dead without zombies or mutants. Just people learning to cope with a new world.