That is what has happened for me. I was diagnosed with stage IV in March 2011. My CA19-9 was over 140,000 and the immediate prognosis was only a few weeks to several months to live. My weight was plummeting, I couldn't eat, it had spread to my liver, and I was horribly sick. Fortunately, I got a doctor who told my wife and I that he'd fight for us for as long as we wanted to fight. His wife is also an oncologist working with clinical trials. They immediately started me on FOLFIRINOX. My markers dropped below 100,000 at the first 4 week blood test and continued to drop dramatically. Tumors on the liver and pancreas continued to shrink. For the past four months my CA19-9 has been stable around 40 (normal is <35) and last Wednesday our oncologist told us it's remission. My weight has returned, up to 165 from a low of 98. For this we are wonderfully thankful that our oncologist continued to work with us even though the weight loss and performance status (quality of life) could have led others to drop or change treatment.
So, yes, I can tell you your sister's response to FOLFIRINOX is not unique and as for what it means, I hope the same as it has been for me... More days, months, and year(s). We still have to work with the damage left by the cancer, and I'm on a maintenance level of FOLFIRINOX. What's next? We don't know. Our oncologist says we are "rewriting the book" on pancreatic cancer. I wish your sister the best and tell her to never give up.