I would imagine you've done this out of concern, but particularly with diaries, if you ever want your stepdaughter to trust you, you need to stop. Just my opinion as a parent. I've managed to raise a very decent 19 year old son without doing that.
That said, any mother with a 13 year old should address with her the issues that she herself faced at 13 years old. Issues of weight, boys, peer pressure, (drinking and drugs weren't faced at 13 in our time but they unfortunately are now, so this is the time for that little chat), sex and how you and your husband feel about pre-marital sex, birth-control and your stance on that, body image, and the entire question of eating disorders.
Now, if your daughter has been purging, (and you don't have to blame it on her mother's death, and indeed, I'd stop blaming her behavior on that, it gives her an excuse for problematic behavior - do you want her to use alcohol and drugs later because you guys are giving her basically "permission" to do so "just" (and I don't mean to sound callous, its horrible that her mother died, but this happens unfortunately, its not something to focus on for the rest of her life every day) because her mother died when she was 8 years old? or isn't it time for her to move on with her life in a positive and productive way. Isn't that what you and your husband expect? Good positive behavior from her? I'd reward that behavior and punish bad behavior no matter what time of year it happens. (And certainly I don't mean to punish an eating disorder. If she continues with difficulty in that, get help for her, although I'm hoping as you focus on your expectation and reward for her good behavior and your positive attitude with her, this other may pass, we'll see).
Anyway, when you talk about the drugs, alcohol, sex, eating disorders, etc, ask candidly if she's "experimented" with any of these, tell her you just want to open a dialogue with her and answer any questions she has and you want to be there for her and let her know what you and her father expect from her behavior wise as she moves into adolescence and to let her know that if she makes a "mistake" that you want her to feel that she can talk to you.
If she doesn't fess up on the purging then, watch her for the next few weeks and if she spends time in the bathroom immediately after meals all the time, tell her you've noticed this and you and her father think she may have a problem and take her to a counselor.
That's my advice at any rate.
Thanks for the advice. i've taken my step daughter to the doctor since i've posted this and fortunately for us, she's only lost a lb since oct. I've kept alot of food on the table that i know she'll enjoy [as well as the rest of us] and havent' noticed her going to the bathroom any after the meals. Overall, this could have been a panic moment for me since i'd not divulged into her private world previously. I too believe young people need their space, however each day i see more and more younger people taking the wrong path and i believe with love and good guidance and a strong faith this will stand them in good stead for the long haul.
As far as mistakes, i'm a firm believer that we've all made our mistakes and it's impossible to keep someone from making their own. What we learn from those mistakes are the key to helping ourselves in the future don't you agree?
You already got some great advice, but I thought I could add to it. Always remember that she will be reluctant to admit anything about an eating disorder, becuase she will feel ashamed and exposed. I would keep a close eye on her after meals, and only broach teh subject if you think she purges again. Otherwise, it may have been a one-time thing where she experimented with the idea. Just trust your instincts and do what you tihnk is best for her. Best wishes!