@raynbow- I think that unless your sister begins to have severe pain or bleeding during her pregnancy, she is on the right path to feeling better. It's safer to take a slow approach to treating an ovarian cyst during pregnancy. In addition, the cyst may resolve on its own without the need for surgery or any other type of treatment.
That being said, I think that it is very important that your sister keep her doctor and ultrasound appointments. This will allow her doctor to closely monitor the cyst, and take a proactive approach should it begin to grow or cause serious problems or pain.