Your question is a very good one and I will work on providing you with some good information and recommendations regarding what is going on with your sister.
Regarding the polycystic ovary syndrome this is something which is mainly treated by focusing on weight loss. Weight loss in addition to healthy exercise and dieting are the mainstay of therapy. There are also certain medications which can be used to decrease some of the side effects of polycystic ovary syndrome.
The extended menstrual cycles which she is having are also putting some stress on her body. This will likely caused her hemoglobin to be low. When hemoglobin is low this can certainly lead to significant fatigue. I think it is also important that she have her iron level is evaluated. It may be necessary to start her on some iron supplementation in order to help support her bone marrow.