A lot of teenagers are slim. I'm slim, and my BMI is on the lower end of normal.. but so are a lot of my non-D friends. I weigh less now than I did pre-dx at 13ish, but a lot of people lose weight in those years. You might like to try a steel needle set. I get less bleeding and bruising with them than with angled sets, which I'm assuming you're using right now (I'm guessing straight in teflon sets would kink if she doesn't have much fat? Mine do).