Your reply is totally valid. I never suffered from anorexia, but my family decided I did. It created quite a rift...

Being told that you are telling lies about having an illness when you don't, being forced to drink weight gain products because you were supposedly untruthful, having friend's parents told of your "illness" and having friends enlisted to watch you and report...

Not fun. I grew up and worked as a model, am still active in sports, and have never had a problem. Two years ago my mother ended up in the hospital, and she begged her doctors and nurses to "inspect" to be sure I wasn't anorexic. They asked with their eyes, I nodded yes, and they smiled at her and listed off the reasons why it was clear I wasn't - I'm in my 30s now, btw.. not a child.

.. back to your story. Yes, you are right. You really are. If this girl is having a problem she might not be forthcoming about it, but she deserves a chance to at least be asked. If she denies it, but her family is still worried, she needs to see medical professionals. Family self diagnosis and treatment of serious illnesses is bullshit. People can make mistakes, but if you put someone in the you deny it because people who have it deny it loop, it can be a very unpleasant place for someone that is just fine.

You are anorexic is something that people who are thin hear ALL the time. It trivializes your illness, which doesn't just mean you are thin, and irritates the rest of us - because if you deny it, well.. that is just what anorexic people do!

Again though, you are right. OP shouldn't ignore it, and shouldn't just take stepdaughters word for it if there is still a concern -- but -- medical professionals need to be involved before the family just starts treating the daughter like she had a disorder.