You are right, it certainly is not fair. I think you are doing tremendously well under the circumstances. It sounds like you are wanting to understand what is happening on a little more advanced level than the average 15 year old.
Have you read anything about breast or pancreatic cancer on the internet? Some people are better off not reading quite so much while some people really want to know what MIGHT be around the next corner.
Let your Mom know that you are vitally interested in what is happening and that you are educating yourself so that you can understand if she chooses to share more.
No one knows what will happen with cancer. It can surprise you time after time. I do know that when my Dad's battle with colon cancer ended I was ready for it. Not to lose my Dad (love and memories never ever die) but to let him go.
And, MANY times chemo and radiation will do the trick.
Maybe the worst thing is not knowing, and even then, luckily, that comes one day at a time.
You hang in there. You sound like a caring person who wants to face things straight on even when they are scary as hell. You can be very very proud of that.
Let us know how things are going - we're here 24-7 and I stop by this page every few days. We care because we've been where you are - you're NOT alone!