Hi, H.. Well, now, let's make sure of what the goal is in this situation. Are you just trying to make sure that the young girl's clothes fit her properly? Or are you trying to get her to step into stuff that might be showing off her curves more than she's comfortable with?
It sounds like she's having a hard time dealing with the changes that are occuring. Maybe she is afraid of the next steps in her development. Try asking her about what makes her uncomfortable about moving up to clothes that are more womanly. Don't just try to make her dress to show off her body. She may not be interested in it. She may have the curves, but still not have the desire to show off her body.
You and her mother may have expectations for her femininity that she isn't comfortable with. My parents and step-parents tried to force me to act and dress a certain way, in conformity to their standards of 40 years before I was born, and not only were they trying to make me look freakish as far as my generation was concerned, but I was not ready to deal with boys, dating, and all that stuff -- they were trying to push me there, and it just messed me up for a long time. So please don't try to pressure this young lady to be someone she's not, or maybe just not ready to be as sexually mature as you think she should be at this age.
Try breaking her in gently. Get her some clothes that fit her better but do not show off her body so much, maybe something that is comfortable, loose and a little flowing, so that she does not feel like she is dressing up like a Barbie doll in spandex.
Some girls are not as fashion-conscious (or fashion-desperate) as the rest of them. Thank God she isn't trying to dress like a stripper like so many of our little girls are these days! Remember that her body image is not going to be fully developed the way an adult's usually is. And praise her for how naturally pretty she is when her clothes fit her properly. Make her feel good about herself, not embarassed for what you think is wrong with the way she looks. It's always better to lift up her self-esteem than to make her feel even more awkward.
Peace,
Syl