First I hope and pray your mother-in-law will be okay. I was diagnosed with neuroendocrine carcinoid cancer. My blood tests came back okay......although my type of pancreatic cancer was hormone producing. It was over producing calcitonin. So this ended up being one the cancer markers for me.
Your oncologists should know what type of cancer and test for those markers associated with that cancer. If you are not satisfied, you may want to seek a second opinion. It never hurts to know exactly what is going on. Frustration is NOT knowing. Know what your dealing with and it will help ease you a bit