Hi, Welcome to HealthcareMagic Forum.
Your sister is suffering from Anovulatory Infertility
. Do not worry, it is treatable but the exact cause has to be ruled out. It can happen if she has PCOS, Hypothyroidism
Imbalance, Obesity. She will need ovulation inducing medicines, if they don't help, the dose will be repeated again. Let her take her medicines as prescribed by her doctor and go for regular follow up.
She should be eating a balanced diet, regular exercises and avoiding stress and practice relaxation techniques
. She needs to have patience though.