This is Dr. David
I am sorry about your sister.
benign ovarian cysts are very very common.
the vast majority of ovarian cysts are benign.
ovarian cancer would show other signs and symptoms more than just a cyst in the ovary.
ovarian cancer would irritate the peritoneum and cause fluid and ascities fluid to be in the pelvis and there would be irregularity of the ovarian cyst and nodules in the pelvis as well
if you continue to worry about cancer, and you are after your menopause, you can always have a hysterectomy to have your uterus and ovaries removed so they will not become cancerous in the future
your family history of cancer doesn't guarantee that you will develop cancer either.
the smart option is to wait on your ovarian cyst and get a repeat ultrasound.
I bet on the repeat ultrasound it will show the same ovarian cyst.
let me know if you have questions.