You are right to worry, and can I just say it's wonderful that you have noticed these behaviours.
I had an eating disorder for years and it started out this way. It's like a slippery slop from that point, so it's great you are on to it now.
She may feel as though she has no control over her life, so her weight is something she can control. She may also have a fear of growing up, which causes her to hate the curves etc that she is developing.
Try not to make a huge issue over the food as that is just a symptom of what's really going on. Encourage her to talk about how she's feeling as the real issue goes deeper than food and weight.
I know it's hard for others to watch someone hurt themselves in that way. You sound like a lovely step mother.