My little sister is 15 and never got her period shes embarassed because our 14 yr old got hers before my sis?

Well my sister is the heaviest one in our family im 155 pounds, 5'5 1/2" and my sister is 329 pounds and 5'6 and 3/4". Could it be because she is heavier and she has a poor diet? I would really appreciate it if you would not be rude. Thanks.