She was so in denial that she made them take him off morphine, because she didn't want him to get addicted so he spent the last week of his life in agonizing, unchecked pain until the last 2 days when I threw a crazy in the hall with the Dr. who was almost in tears himself because he had no say in the matter, I finally talked her into some Demerol.
Denial is a strong emotion, it can protect us from unbearable pain but it can get in the way too.
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 great aunt did a similar thing to my grandfather (brother & sister). Once my mother arrived at her father's deathbed, having traveled from NJ up to Maine, as he lay moaning in excruciating pain, she demanded the doctor give her father pain medication immediately. God knows how long he had suffered with no medication, cancer also. Never heard why the aunt withheld medication from her own dying brother, and she didn't call my mother and grandmother until the very end, barely leaving them time to see him before he died.