I have a theory about Chantal's stunted development and oppositional defiance. We all know that she is socially stunted to around the age of 12-13 and I think that is because that is the age she was when step-dad came into the picture and for the first time ever, someone actually told her fat annoying ass NO. Until then, Grams and Momma Sow let Chantal do what ever she wanted, eat whatever she wanted and there were zero consequences for her actions.
Around 12-13, stepdad came into the picture and wasn't having any of her temper tantrums. Then when Nat was born, there was no way he was going to put Chantal and her antics over the well being of his biological child. This is when she started seriously acting out: supposedly sleeping with older men, the fake suicide attempt, etc. She even admitted being shipped off to the fat-farm and not being allowed to move back home and living in a halfway house / group home. I firmly believe it is because he didn't want her in HIS house any longer. I also think this is why she doesn't visit her family other than holidays or when it is just her and her mom doing something together.
The anger and resentment she has for authority figures, doctors, professionals .... anyone who dares to tell her no .... and doing the direct opposite of what she is told is something a 12 year old would. Unfortunately, Chantal is 36 and thinks it makes her a "quirky rebel" when in fact it just re-emphasizes the fact that Chantal is not wanted by her family and hasn't been since she was a pre-teen (if not even earlier).
Let's face it, if any of us were Chantal's stepdad, we wouldn't want her in our house or lives either. Why should he be burdened with someone else's mistake?