Oh, jeez
This is a major deal for you and your sis to come to terms with. I'm so, so sorry.
okay, in terms of survival - if she doesn't want to go the chemo/surgical route then you are probably not going to have very long - maybe a few months.
When they insert a stent, the tumour can in some cases grow through it, so it will only be effective for a shortish time, depending on the aggressiveness of the tumour. But it will make her much more comfortable for the time being.
Also, sadly it will spread to her other organs if it hasn't already. The only way to stop this would be to have the surgery and the chemo, which kills off any stray cells that might be travelling through her lymph system and bloodstream.
If she isn't happy to have this treatment then you will have her with you until either she is unable to eat any more - they can tube feed for a while, but not indefinitely - or the cancer starts to grow out of control in her lungs or liver or somewhere else, which will gradually make her systems shut down.
I hope this isn't too frank, but I thought you would want the truth about what will likely happen, or you will just be imagining worse things instead iyswim
I hope this helps xx
ps Hospice (and Macmillan) can make her last few months comfortable - Hospice has an outreach team who will come to her in her own home if she doesn't want to leave it. It's worth contacting them asap and getting her all the support you can, they are brilliant at the right meds and so on.