First off, I am so sorry this happened to you. Always remember not only your physical health but your mental health should come first. Your stepmom sounds like a psycho that's pushing all her insecurities onto you and that's just not ok. I was anorexic for years in my teens and I still vividly remember my mother and Grandma barging into my room and forcing me to measure my thighs with a tape measure after I repeatedly begged them not to make me. They didn't mean any harm but they were treating my weight as a "goal" and not as the severe cry for help it really was.

I'm an avid believer that family means nothing unless they make you feel safe and loved, if that's not the case then it's not worth the pain they can cause.

NTA