PET scan before continued chemotherapy or continue chemo before PET scan?

I agree - you need your MIL's doctor to answer questions like these. If the problem is that your MIL tends not to ask them or think of them at the time (which would be completely understandable), somebody needs to go with her to appointments; or, with your MIL's permission you can list questions on an email and contact the doctor that way.

How is she feeling?

There is absolutely no way we can advise from here. Cancer staging is very sophisticated these days and involves more than one expert practice. Often the radiation specialists disagree with the oncologists who disagree with the surgeons. I think that you should discuss with her her own wishes. Pancreatic cancer that is inoperable doesn't have a long survival rate as you will see when you google. At some age (and I as a nurse am THERE--despite 30 year ago hx of breast cancer I do not take mammograms anymore; would not accept chemo anymore) some people have a definitive idea of how they wish to spend their remaining time. Chemo in an elderly person can be a killer faster than cancer, in all truth. Discuss with your MIL and her doctor exactly what her prognosis is before helping her in her own decision as to whether she would like palliative care and hospice when her disease progresses instead of aggressive treatment. Be sure all her paperwork for POA for health care and other care is done. Very sorry for this diagnosis. Some cancers, this being one, and gastric, a few others, can be very quick. Some, such as prostate, grow so slowly (dependent on type) that they are seldom what kills an elderly man. Do get all the information you are able if your MIL wants it. If she does not, and wants just to do as the doctor she trusts suggests, that is an option for her as well. Wishing you luck and hoping you will update us.