Average is just that. I knew someone who died within 4 months, and I know someone else who is alive ten years on.
I was told that survival rates double every ten years, on average, so yes, all the stats are out of date. But it's also age dependant; cancer is our own cells refusing to die (literally - cells are programmed to self destruct, or autophage, and cancerous ones just keep growing and reproducing) so the older someone is, the less aggressive cancer tends to be. A friend's grandmother was diagnosed with breast cancer in her late 80s, and they said other than surgery, there was no reason to treat it, as old age would kick in before the cancer did. They were right and she died in her mid 90s with no cancer involvement.
I was a complete statistical outlier. I was young, and had a huge tumour, so I shouldn't have had the prognosis I ended up with. Very large tumours are very bad news - yet here I am. Statistics can't tell you individual outcomes.
The statistics are general. They don't determine what your family member will have. We have a family member who was given 5 years for his particular cancer, and is still with us. He was given that in 2000 and hasn't had any especially major interventions, just 2 separate rounds of chemo and one of radiotherapy, all split by years.
Cancer is bizarre and unfathomable. But realistically, someone with Stage IV is unlikely to be alive in ten years, and 5 is a good one, too.
There is one other thing I'd say. Loved ones and friends tend to be very, very upbeat when someone is diagnosed with something like this. That can be very isolating. Someone saying, this is really shit and I am so very sorry that this is really shit can be a greater comfort because it feels less lonely. It feels like a wall goes up and they don't want to hear your fear, or that you know it could be very, very bad news and your life far shorter. You start to feel silo'd in by people who just don't get it.
There is a Facebook group, Younger Women With Breast Cancer, for the under 45s. It helped me immensely as we're all in the same boat, and women who are that bit further ahead supported those who were newly diagnosed. All my questions about chemo were answered, and at 2 am when distresses and terrified, a loving group of women who got it were there. Maggies Centres are also vital for your family member. You need support, and at least in my case, I felt unable to share the real fear and distress with my loved ones as they were suffering as much as I was, in different ways.
There is one other thing I found helped me. We are all dying. Nobody gets out of life alive. Think of those you love in their 80s - they are not going to live forever, either, yet there is something about a cancer diagnosis that terrifies because the very human denial of our own mortality is ripped away from us, with it. The important thing is not to waste any of the good time you are guaranteed to have today, agonising about the fear of death. You need to plan lovely things in the here and now. Go on the lovely weekend somewhere you always wanted to. Book Giffords Circus, Christmas Light Festival at Kew, a week on the island you loved as a kid and want to share with your own. I have never valued my one, amazing, precious live more, nor appreciated it better, than since I was diagnosed. We will all die. All of us. So embrace the life we have and show this person how much you love them - and not just this person, either. Life is fleeting and amazing and precious, and cancer isn't really anything but a stark reminder of all of that. So use it.
There's also something in the NHS Constitution called The Right To Choose. If your family member wants to, she can be seen at the Royal Marsden in London, which is the most phenomenal care, and all NHS. Everyone has the right to have care transferred down to them and they are an international centre of clinical excellence. I have been on around 3 trials with them, and my care is amazing as a result - effectively, the trial company fund it, so I have ongoing care years after treatment ended, because they want to pinpoint exactly when, and if, the cancer spreads in both the treated and the control groups. If there are new treatments, which are worthwhile, the Marsden will know about them. Worth considering, at least.
Love to you. As above, I genuinely believe that it's often harder to watch, than to suffer, when you love the patient. Maggies help families, too. (My kids had never had so much spoiling!) You need to charge your own emotional batteries, if you are to support the person you love, so it's not selfish at all. Take time to get the help you need for yourself, and I am so very sorry you are coping with this fucking horrible thing.