Her mom plays a huge role in making her a rotten egg. She got carelessly knocked up young like a wild animal and didn’t take responsibility- foisted her bundle of burden upon Grandma.
Chantal is addicted to attention which can never fulfill her because the attention she ultimately seeks is the attention she needed from her mother when she was young. That ship has sailed and nothing can bring it back to port.
The food addiction might be as simple as missing the breastmilk from the only one in life who should have loved her yet gave her away. Now she’ll cram anything in her mouth but it’s not the same and never enough.
Grandma probably did the best she could. Chantal has said Grams complained about her eating too much. I can picture a young Chantal, a little chunker moaning over spaghetti at the dinner table. “Shhh!” Grandma would say. “MmmmMMMMmmm!!!” Chantal would repeat. “I said SHHH!” Grandma would reply.
It must have hurt her deeply to see her mom start afresh and do much better. Honestly, I’d be jealous of Nat, too.
Her nurture was a little messed up (granted, other people have it far worse and turn out far better) but her nature has a big part in this as well: