The above posters are correct in saying that there is no way to know how long she has left.
I lost my maternal grandmother this year, and I am still mourning her. I lost her not long after my own cancer diagnosis (endometrial and in treatment). Cancer is sneaky, and can lay silent and quiet for a while, or it can charge head on, fast and furious. It is so difficult to not know what will happen. Is she alert and coherent still? Does she undestand her diagnosis/prognosis?? What are here end-of-life wishes?? Is she receiving any type of treatment right now???
Just continue talking to her as if she completely understands, even if you don't think she can. There is no way to know if she will still be "herself" or if the cancer in her brain will change her behavior. I can't tell you what to do, obviously, and so much depends on what her needs are and I don't know them. Just be there for her, and love her like you always have. Give her the comforts of home if she is in the hospital. I promised my grandma I would tweeze her chin hairs if she ever couldn't do it...and I sure did do it right there in the hospital. Sounds stupid, but she always made me promise...
Consider hospice. My grandmother was placed on hospice hours before her death, because my family (my mom, aunts and uncles-my grandma's kids) did NOT understand hospice. It didn't matter that I had given them a lot of information, or that a PA in the family gave the SAME information I did, etc. They just sat and thought she would be okay, that she was just weak and needed to "eat". They were in total denial, and did NOT understand her condition at all. Even though I told my mom in private that she was dying. The day before she died, my aunt went to see her at lunch and I walked into her room to find her stuffing bread soaked in soup into my grandma's mouth. My grandma aspirated all that fluid. No one listened, until the RN in charge had a hospice provider come in, but they waited till all the kids could be there to make a decision. They waited four days, signed her up, and a few hours later, she died. She had IV's going in her swollen arms, Potassium in one of them which is very painful, she had SCD's on her legs, TED hose on, etc. My family wanted my 60 pound grandma, all contracted and comatose, to go for multiple scans to see "what was wrong with her". She was dying!!! I will never forget how horrible those days were, until she was put on hospice, the IV's were stopped, she was given ample morphine and ativan, and no longer shaking from pain. I am thankful she at least passed away comfortable, pain-free and peaceful.
I am not trying to give you my story to scare you, I just wanted you to know that hospice is available, and I have had nothing but excellent care from them on both a personal and professional basis. They will be available for your half-sister's mom to give her pain and symptom relief, and they will be available for you and your family to offer grief counseling, etc. It truly offers kind and compassionate alternative in a very emotional and traumatic time. They will do all they can to keep mom pain free and comfortable during her last days. I wish you and your family peace, and I will pray for all of you. If you want someone to talk to, please feel free to PM me.
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