I'm sorry your sister is dealing with this.
The CA125 is notorious for elevated results when there is no cancer. A friend and I had the exact same type of benign complex cyst and her CA125 was over 1000 and mine was normal at 30-something.
A frozen section biopsy of the cyst tissue can be done during surgery and there should be no need to remove the cyst if the result is negative / benign. You are right to be concerned because, unfortunately, far too many women lose an ovary or ovaries for benign ovarian cysts. Removal of even one ovary has been shown to cause permanent harm due to the loss of ovarian hormones which are essential our whole lives for every aspect of good health.
I'm not sure how to go about finding a surgeon with good cystectomy skills. Any surgeon who balks at the mention or insistence of cystectomy for a benign cyst should be crossed off the list. But aside from that, I don't have any tips. The non-profit HERS Foundation may have some suggestions.
Regardless of your sister's surgeon selection, she will want to protect her organs by ensuring the surgical consent form is explicit about what may and may not be removed and under what circumstances. Most gynecologic consent forms give far too much leeway to the surgeon to remove whatever he or she desires. That is part of the reason female organ removal surgeries (hysterectomy, oophorectomy, salpingectomy) are far more prevalent than they should be. About 90% of female organ removals are unwarranted. Yet they continue because they are lucrative and insurance companies don't care if healthy organs are removed. :(
Please keep us posted. I wish the best for your sister.
I've read that ascites can be concerning but they aren't necessarily indicative of cancer. Best of luck with tomorrow's appointment.