hello and thank you for asking HCM, I understand your concern. There are a lot of reasons why your sister in law could have amenorrhea. The first reason you should consider is pregnancy if she is sexually active. The other reasons could be health problems which could lead to hormonal imbalance, for example thyroid malfunction, polycystic ovary syndrome, premature menopause. Lifestile factors can also influence your period (low body weight, stress, excessive excercise). My advice is to visit her gynecologist to make a detail examination because she is to young to lose her period. Most of these reasons can be treated. I hope this answer was helpful to you.