Teen with uneven breast development

It is perfectly normal to have different size/shape breasts.  I would wait until her breasts fully develop.  If at that time there is a concern about asymmetry, then:
1) You can reduce the larger breast with liposuction
2) You can enlarge the smaller one with fat grafting or an implant
3) You can enlarge both but use different size implants
I would visit with a board certified plastic surgeon at that time to discuss your options in more detail.

While most believe that a woman's breasts stop developing by about 18 or 19 years old, they actually continue to develop and change throughout a woman's life. Pregnancy, hormonal changes and aging can all make the breasts change, too. Asymmetry is totally natural and not something you should be worried about. No woman's breasts are symmetrical. Think of breasts as sisters, not twins. 

Breast development continues throughout a woman's life, with a first stage of maturity achieved at 19-20 years old. Asymmetry is also the norm for women, as no breasts are perfectly symmetrical in size and/or shape. For these reasons, I recommend your daughter wait until she is 20 or so before considering surgery, as she still has many years of development to go.