42 year old daughter has terminal renal cell carcinoma, spread to lymph nodes in neck and one "spot"

Hello,
I am in great distress. My 42 year old daughter has terminal cancer (RCC) . She is married to a wonderful man and has three sons (20, 18, and 9) I am heart-sick and feel so much anguish and fear that she will not be able to finish raising the 9 y-o. That is her biggest heartache also. She had one kidney removed over two years ago; I cared for the boys and their home while my daughter recovered from the surgery. They are self-employed farmers, able to hire the outside work done, and my son-in-law stayed inside the house for two weeks seeing to my daughter. I began to notice that when the older boys came around by their mother (from another marriage) they were either ignored or dismissed. It was like my daughter and her husband were in their own private world. I could see the hurt and confusion in the older boys, but this was a time to keep my daughter calm and surrounded by love, and not INTERFER with comments. But things have escalated to a point where I am uncomfortable, sometimes hurt in my heart, and times when I see the children yearning for some time and affection from their mother. In the meantime, my daughter is receiving many gifts from her husband - android cell phone, coffee brewing station, laptop computer, and smaller items. When she saw an expensive new car she wanted, in no time they had purchased it. They are going, the two of them, to Jamaica later this month and she is planning a summer vacation to the southeast coast, the family will stay in a rental home for a week. They are not rich, but probably financially okay, and anyway that is not the issue. What is concerning me is my daughter seems to be developing a narccistic personality - she is always right, she is possessive, she can say whatever hurtful thing she wants to anyone and her husband has stated that no one is talk to her in any upsetting way (husband tells me she talks mean and hurtful to him but he lets it roll on by). There seems to be a strangeness between my daughter and I and recently when she was accusing me of something I was not guilty of, I stood up for myself and tried to talk with her about the situation. She didn't speak to me for 2 1/2 weeks, wouldn't return my phone calls, my pleas to talk things over and my expressions of love. I have been sick to my stomach since this happened. I finally called her husband to see if he could suggest something for me to do. Shortly thereafter she called me on the phone but when I tried to explain to her my feelings, she began screaming at me, threatening to hang up. I lost my temper and yelled back, then began to cry and apologize. Her voice full of hate and mine full of sadness. Now I do not know which way to think: (1) do I back off, apologize, and just watch her behavior escalate into more self-centeredness. I am walking on egg shells around her which makes it difficult to enjoy our time together. (2) Or do I find a new way to approach this, to try to gently guide her away from this behavior? I am at a loss to think of ways to do this, and maybe I should forget the idea altogether. She may some day look inside herself, and become troubled that she has hurt others. I wonder sometimes if the medication is affecting her behavior. That could be very possible, plus the emotional side to this is extremely difficult for my daughter, for all of us. The illness is changing us all. Can anything be done to keep my daughter from what I consider ruinous behavior As I write this I can see that this type of behavior may be "normal," maybe a given as a side effect of medication as well as the horror of the future. My wish is for my daughter to have many quality days and years ahead. We have not given up hope as new medications are being discovered every day. Someone could deliver a miracle in a pill or technique. In any case, I want to have a loving relationship with my daughter, but find it very difficult when she acts mean and hurtful. My own health, mental and physical, is at a very low place and I would be making a mistake for my sake to not bring this out in the open to look at it and ask for help. Can you help me? I'm open to listening and then doing what I need to do for my daughter. She is my oldest of four, has been a leader in our family, likes to have fun and can herself be a lot of fun. Down under the pain she has a good heart. She has done a fantastic job with her sons (and I do tell her that, told her when we had the telephone fight). Her way of dealing with this often is to buckle down and plow through it. Kind of a bull-type strength. And sometimes she tries to deny it is happening. She has said she hasn't given up hope, that she plans to be here a good many years. When the doctor came out of surgery to report he had successfully removed her kidney, he said we should think in terms of five years, and if in five years my daughter is still here, then we think about another five years. Well, that was shocking, but now I realize a lot can happen medically in five years, new discoveries are happening/being worked on every day. HOPE is a big think with us, with all cancer patients I imagine. If you give up HOPE, then what is there.
Thanks for reading this, if I have sent it to the right place. Otherwise, advise me where I can search for help next.