Someone Needs To Tell Her?

My mom has stage four ovarian cancer, which to this day she will not acknowledge.  
How do you handle a situation where I really believe the cancer is effecting her brain function?  
I mean my mom has always been by nature a gabber, gossip, tell the whole world kind of person. It made for a very rough childhood, I mean to have expectations put on you to be a certain way, dress a certain way, then when those expectations weren't met, the whole world would know your failures before you even got off the school bus. But after being diagnosed, then a colostomy bag after surgery, she changed into a considerably quiet human being. A considerate human being.    
I could talk to her without her blowing up or yelling at me , "what you should've done."  
However, she chose to put the cancer in God's hands, no chemo..no radiation, which I respect anybody's choice on how they handle their sicknesses, but now because she won't even say the word cancer, Im left as a daughter to discern where we are in this journey.  
She's jumping all over people, and calling people she hasn't talked to in years, and that would be fine, but I found out she's bad mouthing everyone in the family, who have absolutely done nothing to her.  
Now everyone that could help won't because they are staying on the surface of the situation and blaming it on that's just the way she is.  
Well, could be, but something in my heart is telling me this change in behavior, the lack of empathy and control is from the cancer. But I pray because I just don't know how to approach this issue. Id hate to get all defensive and combative with her if she's really not in her right mind. I'd never forgive myself.    
How do you approach something like this when she won't even acknowledge all of her health issues are from the cancer? 
I swear when she lays in bed for three days ailing, she literally says, " I just don't know what's causing this," then when I say, mom...it's the cancer, she blows up. I don't know, I just really don't know, I pray God gives us the ability to handle this right, because heaven forbid we handle it wrong and she passes suddenly and we realize it wasn't her fault.