The heightened tumor marker could be due to the swelling. My CA125 level shot up after surgery, but they drained 3L of fluid out of my abdominal region and it tested negative for cancer cells.
I think it's a good idea to get a second opinion as you need to find out just what is going on. Had your sister gone into remission after her operation and chemo and, if so, has the cancer now returned? The fact that her tumour markers are very high would suggest the cancer has returned, but has this been confirmed by other tests? What exactly is causing the fluid in her stomach and why can't it be removed without causing infection or bleeding?
Your sister needs to find out the answers to these and probably other questions, and to discuss with her oncologist what should now be done, what treatment is best. Getting a second opinion from another oncologist seems to be a wise move in the circumstances. I hope your sister gets the answers and treatment she needs.