You could be talking about my dd, who is mixed race, twelve, a bit plump, curvy, and wears a d cup bra already.
It's very hard for them to resist the unhealthy eating habits many of their friends will have.
My dd started going to fried chicken shops on the way home from school with her friends when she started secondary. Also buying cans of drinks. In a really short space of time she PILED on weight and is only now, slowly, starting to slim down a bit.
I've talked to her a lot about 'empty calories' - explaining how having even a couple of cans a week of fizzy drink on a regular basis can, over the course of a year result in significant weight gain. Also about how it's important to eat regularly, and to eat food with fibre, plus fruit and veg. Protein at EVERY meal.
She refuses to eat breakfast now, so I buy her a yogurt drink to have on her way to school. She comes home hungry and I encourage her to make herself scrambled eggs, or cold chicken, rather than crisps and sweets.
I'm thinking about getting her to watch Super Size Me with me: en.wikipedia.org/wiki/Super_size_me which will put her right off junk food.
I've discouraged her from going to chicken shops by discussing animal welfare issues (these shops use very low grade factory farmed meat. Sometimes you can see the marks of the abcesses the chickens have developed from being crowded into small cages before being slaughtered). Also discussed shops reusing oil too many times.
Good luck with your step daughter!