I was 22 when I was diagnosed with rectal cancer, and the original protocol called for radiation and chemotheraphy. After finding out about the effects of both, my family and I became concerned not only about fertility issues, but early menopause as well. We eventually opted only to do chemotheraphy, and I sit here today 7 months after chemotheraphy and 1 year after surgery clean and healthy!
But to get to your question... before I started treatments, we spoke entensively with the oncologist about options for saving my eggs, ovaries, etc. While I didn't wind up doing any of them, he did give us a couple of names. One doctor is in NY Presbyterian - Cornell, right across the street from Sloan Kettering, and there were two others nearby in NYC who I spoke with as well. The other two doctors were willing to work with me and give me a "discount" to freeze my eggs because I was a cancer patient and my insurance wouldn't cover it. I think one was Dr. Vidali, Dr. Husami (??) (they worked together), and the other at NYPH was Dr. Glenn Schattman. Let me know if you need more information and I'll see if I can find it for you.
Good luck to you and your sister!