Sarcasm is an adjusted stress reaction.

IIRC from her LJ spoiler-thread, the author acknowledges that the relationship is not what you'd call healthy in anything but a zombie-infested post-zombipocalypse; something along the lines of pressure-cooker environment, emotionally distant "parents," and no real options for relationships outside their tiny, enclosed home environment until after they'd already established their pairing. (Sort of a Flowers In The Attic vibe, only the author didn't make that comparison.)

If it helps that it's not presented as a "incest, yays" and more "broken world, broken people, making the best of what they're dealt." (Which doesn't mean you should read the book either way, but if it helps in a... *waves hands* thingie. Bah, I cannot brain. Bedtime...)