I'm really sorry to hear about your MIL.
I don't have any personal experience re the cancer side of this but I was hospitalised for 2 weeks and off work for another 3 weeks recovering having had a stent put in to relieve a blocked bile duct.
It was the most agonising thing I've ever experienced to date and my medical file is pretty thick!! I actually thought I was going to die and it took forever for them to get a proper diagnosis. I lay in agony in the hospital for 12 days while they ran various tests and just messed me about really and when they finally put in the stent, I was like a different person within 48 hours.
It still took me a few weeks to recover, but the agonising pain was gone, the jaundice was much improved within 48 hours and it just made a HUGE difference. The stent is still in place and touch wood, I have not had any further problems since and that was roughly about 6/7 years ago.
I can understand your apprehension re your family thinking this will prolong your MIL's life and I think you are right to feel this way. I think it will make her more comfortable but being realistic I think it's unlikely to prolong her life.
I've lost several friends and family to cancer over the years and I've never heard of anyone surviving for more than 6 months with a secondary cancer from when it was diagnosed. I hope I'm wrong but I think you should perhaps try and prepare your family slightly for the worst so they are not building up false hopes and try to enjoy the time you have left together as much as possible.
My thoughts are with you and your family.