Is the cancer back after all this time?

Hmmm. I had ovarian cancer so I'm pretty familiar with it (if that's what your sister had. Hard to say from your description if it's uterine cancer or ovarian cancer). She might have fluid because she's had a cyst rupture, for example, or because she has endometriosis. But at this point, the specialist she will see is I'm sure a gynecologic oncologist, and s/he'll be able to make a better determination. Your sister needs to be followed by a gyn-onc from now on to check for recurrences. The heavy bleeding you've described doesn't sound to me like an ovarian cancer symptom, though it sounds more like a uterine problem, and it could be something benign. I know this waiting is a real nightmare. Good luck!