Excused for Bad Behavior?

♻️Lexi
My grandmother has lung cancer and is beginning to suffer from dementia. She is 79 years old, full of onery and misery (so says my other family members). She's said maybe one hurtful thing to me in my entire life, other than that, she has always been my sweet grandma, even till this day. I have yet to witness her being ugly. Every time my mom and aunt make a comment about how much she stresses them out, or how they can't wait for her to pass, I get sick to my stomach. I always viewed it as, regardless of her actions, she is still a human being who's been the root of our entire family. 
 That being said, my mom has become sick. She's suffering from a neurological disorder that affects the pain receptors to her face, on top of fibromyalgia, diabetes and other medical issues she's developed. She's been notorious for saying some really degrading things from my personal experience, and now that I'm older I choose not to tolerate it and I call her out on it. I love her and I respect her, and I calmly try to redirect the conversations to where she can understand what she said was wrong and we move on. 
My aunt called me yesterday and notified me of the extent of my mother's sickness and suggested I just be careful of what I say, yet she refuses to do the same to her mother (my grandma). 
Now I realize what I preach to them about just being nice to my grandma, is reflecting on me and my mother. I'm at a tie with if sickness and medical condition is excusable for ones actions. 
In one hand. I don't see my grandmother as often and I can easily dismiss any remarks because of her state, but in the other, I've been hurt multiple times by my mother and I have a difficult time looking over her remarks just because of the state she's in. 
I don't really need personal advice or anything, I can manage my own views, but I am curious to know from others-
Should bad behavior be excused due to physical conditions or is there no excuse for bad behavior?