There is something a little off about her, although whatever it is, it is not as egregious as it is in Clotso's case, and it manifests itself differently.
Frankly, her mom is a shade or two kookoo too, although not enough to make her stand out in a crowd. But she has raised a few eyebrows at times, whether trolling her fatso daughter with fat jokes or handing her a bowl of fudge when she was on a diet. Not saying she is any worse than anyone else; we all have our kookiness, I reckon. But something happened to Clotso that was pretty severe, and it may very well have affected her little half-sister, thirteen years Chantal's junior.
Even Buster seems a tad neurotic. Most dogs are a lil more chill.
Insofar as Nat might have some issues, one could wonder who was the bigger influence in making her that way? Kooky mom or the three-ton bruiser Chantal? Who would a three year old girl have feared more? Kind lady who pampers her, or the fat teenaged interloper who comes into her house to stay, knocks her off her own sofa, and changes the channels away? I'm no child psychologist (at all), but I can see how Chantal might have influenced little sister's development (to the point where little sister had to bulk up and work out). This is only thunkposting, we will never know for sure.
But yeah, the whole family seems a little challenged, except New Dad who has a steady job and has held at least the "new family" part together for 25 years and seems even willing to tolerate step-daughter. That dude seems upstanding, even if I would quibble about his taste in chicks. So if it turns out to be something genetic, look to mom.
Unless you're a Kiwi Farmer. 
