Hi,
Welcome to the forum. Sorry to hear that your step daughter is having such a hard time with her diabetes. I was diagnosed with type 1 diabetes last year when I was 19. As your step daughter is 15 she is still going through adolescence and changing hormones at these time can make it much harder to control blood sugar.Is she on injections or a pump? A few things that may help with her control are making sure that her basal insulin is right and also checking that her insulin to carbohydrate ratios are correct too. Think Like A Pancreas is a great book that explains how to do this and has helped me a lot. It is available from amazon. When I was diagnosed last year my blood sugar would swing around from very high to very low all the time. Learning how to carb count accurately and match my insulin to my carbohydrate intake as well as learning how to time my insulin correctly based on the GI and GL of the food I am eating has helped. Getting my Lantus dose right has been a huge challenge(and one that I'm still working on)but if you can manage to get the basal dose right it improves the highs and lows significantly.
Other books that may help you are Type 1 Diabetes in Children Adolescents and Young Adults
Carbs and Cals (this is a picture carb counting book that is also available as an app for the iphone or iPod touch). Both of these are also available from Amazon.
If your step daughter is on injections, like me, then a half unit pen ,if she doesn't already have one, is a really handy tool and has really helped reduce a lot of my highs and lows.
Hope this helps. PM me any time if you ever want to ask anything.