My dad had stage 4 lung cancer which had spread to most of his vital organs and I was lucky enough to have moved up from London back to Scotland before I found this out; this, at least, allowed me to see him and my mum whilst the inevitable happened.
My dad was coherent and lucid to the last few weeks when things went south. We would talk about various things - family, my wife and as-yet unborn child, the weather, football - but he would always, always come back to the fact that I should live my life as best as I possibly could. Not because he was dying but because he wanted the absolute best for me.
Your mum will be the same, I'm sure. Chemo is harsh on the body and she'll need support but you also need to work out how you see the rest of your life panning out.
I don't think anyone can tell you what to do. Whatever you end up deciding to do is the right thing to do.