Why Damian is the only one normal in their family? （/TДT)/

Maybe he just hasn't been zombiefied yet

Lol XD

In all seriousness, it seems like the mother might be terrified of her husband. She doesn't seem like a zombie but she does seem mentally unstable. The son/older brother, I'm not 100% sure. As an elder sibling, I can't get past how cold it is to let more Damien struggle, but if he's also been traumatized and/or experimented on or something, I guess I have to feel bad for him too.