That doesn't sound like denial, it sounds like your MIL was/is a nutcase. Denying a dying cancer patient pain medication? Dogs receive more compassion and better treatment.
It's interesting that you don't talk about how your husband did or did not "handle" his mother and his father's illness, but it seems you were the one trying to talk some sense into the MIL. It also seems odd that the doctor involved in the case had no say over his patient's treatment.
My husband tried to reason with her but it didn't help, hence the hissy fit I had in front of the Dr. which mortified her and she was desparate to shut me up. The Dr. had to go by her wishes, I guess, as my FIL was pretty out of it. I had to go behind her back and call my SIL and my FIL's sister and tell them that if they wanted to see him again they better come. I had gotten firm orders not to tell anyone to come home when I mentioned that we should call them. It was like she was ashamed that he had cancer or something. It was a pretty rough time but I found out that when I have to be I can be one strong woman.
On the very wonderful side, we farm and there was a lot that I couldn't do on the farm while my husband was with his father for most of 2 months, one weekend all his farmer friends came with their tractors and did all the spring work that needed to be done before planting.