We didn't know enough to get things faster when my husband was first having problems. They spent months and months just coming up with a diagnosis... and it turned out to be wrong.
Dan's doctor prescribed PT for his back pain... and when that didn't help, and we couldn't get a close appointment with the doctor, his PT actually told him to go to urgent care. Said it was urgent because Dan couldn't sleep because of the pain. And the urgent care doctor was the first to order a scan that showed "something" on the pancreas, lungs, bones (back) and that at least got us in to see an oncologist. It was a frustrating time of waiting so I can understand what you are going through.
There were months and months of scans, doctor visits, etc... and they decided it was all "consistent" with lung cancer. That is a word to watch out for as they did not do the necessary biopsies or CA19-9 tumor markers to determine that it was in fact pancreatic cancer until more than 6 months later.
When you've had no health issues in 70 years, you don't know the right questions to ask and depend too much on the doctors and their schedules. All I can say is keep pushing to try to speed things along. Our hospital has a "navigator" for people with cancer and she has been a great help in trying to take care of things when we get put on hold. Not sure if your hospital has navigators, but if they do... definitely worth getting one to help.
Good luck to you and your sister in law.