Extensive small cell lung cancer with mets to the brain

So 2 days after Thanksgiving my mom was sent to the hospital because she couldn't remember how to write her name or answer her phone at work. By the end of the night they were telling us that she had extensive (stage IV) lung cancer that had spread to her brain. They were able to take out one of the tumors in her brain but not all of them. She does have one sight on her lung as well. I work in a semi-clinical field paying Long Term Disability claims and I see things like this all the time. If it were just a claim that had come across my desk I would have that gut awful feeling that I would be doing a life check on this patient in 6 months. Well when we spoke to the oncologist she suggested chemo therapy. But also mentioned that this type of cancer does come back. I feel like I am the only one that realizes that she will die from this. Or maybe she won't and I am being negative?? I am doubting myself. My father and brother are sad but not understanding that this will be her demise. I have heard the dr. say "end stage" cancer, so I think they would understand what that means. maybe I don't understand what that means. I always thought I did. So the chemo could work and she could survive? Really what are the chances of that. I am a pragmatic person and like to know things blunt. I understand theres a healthy way to be optimistic and I would like to be that person too. I just want the whole picture. I want the optimistic, the pesimistic and the realistic information. I just feel like if we do not have any idea of how long we have with her then we won't savor every minute as tho we may have another and another. Example, I asked for a huge Christmas since this will probably be our last together, my brother thinks I was crazy for saying that and got very upset with me for being so negative. I think I was being realistic. I am not trying to be negative. My mother is the love of my life. She is the closest person to me, and I will not be able to breathe with out her. I really won't. I can barely breathe now, just thinking of my life with out her in it. I am not done being her child yet. I am a mother of 3 and that goes out the window when I am talking to my mum. She is my mum and I need her to stay being my mum. I am pulling the kid card, and I want to stay her child. I don't want to lose her. I can't bare it.

I just don't want to be blind sided into thinking she is going to be alright and then re-live the day that we found out she had brain cancer. I can't do that all over again, I can't I know it. I just love her and I want to be prepared for the worst. Is it unreasonable to say that chemo will give her a few months, but that it will reoccur and that will be her ultimate demise in my best non-clincial opinion of a year????