C.C.
She should have her thyroid checked. (You can google "symptoms of hypothyroid" for more information, and also see www.stopthethyroidmadness.com.) I had my period for about 6 months straight before any of my doctors figured it out - and it's a simple blood test! If your sister does have low thyroid, it is easy to treat. Once-daily medication, and the medication itself has no side effects.
Best of luck, and please let us know what happens!