What can you say? Just let her know you love her! And again make this time you have the best you can possibly make it. Like I said some of us don't get a chance to know what it is that will be our end. Maybe that is better but some people don't live their life then it is gone. Try to just be there for her. And if she doesn't want chemo and radiation support her. I would not want my last months on earth being sick like that unless I knew it would help. My thoughts and healing prayer's are with you both.
I have a friend who lost her sister to a rare cancer some years back her sister was only 34. I spent time right after with my friend for a few weeks. She loved and supported her sister like you are and in the end she was glad they just lived with the time they had left. But she did not like seeing all the treatment her sister went through it was not good.