This is a very, very difficult subject and you are not going to find the answer in any of these posts. I stopped smoking at the same time as my friend - she got lung cancer and I didn't. She started smoking again, I didn't.
Others have addressed you already on the lottery of lung cancer, lung cancer patients who have never smoked etc. All of that is right but your Mum does smoke and that is the entire point of your post. It's clear you've never smoked. You seem to be angry not because your Mum got a horrible disease and it wasn't fair but because she did it to herself. Maybe she did, maybe she didn't but whatever caused it I doubt she's a truly evil person and therefore she does not deserve it.
Do you know what I mean by the "it won't happen to me" syndrome? As a smoker one does appreciate the risks on an intellectual level but do you honestly believe that any smoker here thought they'd actually get lung cancer when they were smoking that cigarette? Have you ever driven your car too fast? You're sure that risk's Ok because you'll never knock someone over will you - it's not going to happen to you, is it? If you did would you expect your Mum's support or would you expect her to have so much anger that you remained unforgiven till the day you die?
It may not seem like it but I do sympathise with you. I don't mean to patronise you but you are very young and you are going to have to deal with this quickly. I could go on for ages but that won't help. Your Mum is most likely going to die. Are you going to be loving and supportive (your Dad could probably do without this) or are you going to make this more unbearable than it already is?
You have no control over your Mum's destiny but you do over yours. You've judged your Mum as weak but you've shown that you are too. Let go of this or you'll spend the rest of your life regretting it and there will be ramifications you haven't even contemplated yet.
I really do wish you the very best of luck.
Dee