LADIES: My niece is 15 and is not even an AA cup. Help?

Some woman and girls have small breasts, but she should get up to an A by the time she's 18. I'm 13 and an A, but I'm only a late bloomer to my classmates. If she doesn't get her period she could get up a few sizes, but if not pregnancy gains you 2 sizes or so. She's normal, and help her to understand that.