I had fluid in my pelvis and a huge blood clot in my abdomen after my surgery back in February and was in hospital for six weeks altogether.
The blood clot was from a rather large amount of internal bleeding, I had to have a four unit blood transfusion, and the fluid, no one really knows where that came from!!
I had abdominal drains put in for the blood clot and had the fluid draining under general anaesthetic as they had to do that through a rather uncomfortable area. In the end the fluid just disappeared, which was a relief, as I was about to have a drain put in my butt cheek!!!
I hope your sister begins to feel better very soon, I'm sure once it starts draining out she will. Having fluid internally, especially if it is infected is pretty painful, and personally I was totally out of it for at least a month while they tried to work out how to drain mine.
Should also add that since getting out of hospital in April I have felt pretty awesome, so there is light at the end off the tunnel. I have one ongoing problem with a fistula draining from my stomach, but hey I've survived worse!
Big hugs to you and your sister, hang in there!