V.D.
My mother is currently going through this except hers is the size of a grapefruit they've told her. They found it during her yearly exam. For my mother they have done an ultrasound, and a type of cat scan. It's hard to say what it is for sure. For my mother they've decided to go in and remove it because of the size. Ovarian cancer can happen at an early age. I've seen it happen before. That's not to say that your sister has it, but I would suggest going to a specialist and take care of it asap so that if it is they can stop it before it spreads.
Hopefully it'll be nothing more than a cyst. Good luck. Hang in there.