thanks for using health care magic.
Irregular menses is common in young girls 1 to 5 years after they started having their periods and is usually due to a condition known as anovulatory cycle
because their brain to ovary system responsible for menstrual cycle is not fully matured. She does not need to be scared to see a doctor because its only after an evaluation by the doctor that a definite answer will be obtained and an appropriate plan instituted.
So i will advise you counsel your sister and let her see a gynecologist for a review.
Best regards and hope this helps