My 15 year old cousin is upset that her 9 year old sister sister has gotten her period and she hasn't..?

She says she feels like she cant talk to anyone from her immediate family about it because they are all making a big deal of her little sister 'coming of age' and she feels really embarrassed that she is so much older and still doesn't have her's yet. I told her people develop at different rates but she is still upset and is starting to wonder if she is infertile or something. how can i cheer her up?