I'm very sorry to hear what you and your family are going through..
It's a very tough place to be and you just want direct and honest answers...the thing is that no one person is the same and we all experience death as an individual.
Is your Sister in law on hospice care? If so, there is a great book that I was given that outlines (roughly) the different stages of the end.
My Dad passed away a little over a year ago from Small Cell and had several metasis to his brain...He went through several months of Chemo and Radiation but after 6 months or so of regression, the cancer came back quicker and more aggressive then previously...he had several more brain tumors that had to be controlled and more frequent seizures until the doctors had them under control.
My Dad experienced different symptoms at the end...loss of appetite and thirst, more and more frequent sleeping, irritable (even more then he was known for
). He was unsteady due to the tumors, forgetful, loss interest in things like tv, newspapers and radio. his sleeping would get more intense and he started sleeping 14-16 hours out of the day. His hands and feet would get cold ...
this lasted for a few weeks, until one day he had an energy surge (which can be a normal thing), I took him by my Mom's, he ate 2 Burger King burgers and fries, was up and talking and arguing with me.
My parents were divorced for many years prior, but they remained good friends. He wanted to stay at my Mom's for the night and I needed a good night out...but when I came to pick him up the next day,he slipped into a coma - possibly from a siezure.
This may sound terrible, but in actuality, it was a blessing. My Dad slept while his body wound down and prepared for it's transition. No pain, no worry. He died early the next morning.
Your sister in laws experience maybe a completely different transition, but there is an abundance of online info about what to expect.
For me, knowing what to expect helped me keep a calm head during the process and to plan out anything I needed to do to make certain my Dad was always comfy.
Hope this helps...