I need some advice in regards to my fathers recent cancer diagnoses.
The jist of the following message is this; my father is a heavy pot smoker and alcoholic. Is there any point in me trying to get him to stop? If so, how on earth do I do it?

I found out a few days ago my father has cancer of the mouth. This is not a surprise as he is a heavy smoker and alcoholic. (At least 30+ cigarettes/weed joints a day, + at least 7 cans of beer a day). The prognosis is good and it is highly probably he will survive.

I don't live at home, however my father lives with his alcoholic, pot-smoking wife who encourages his smoking and drinking, and also with my stepbrother (also a heavy drinker and pot smoker) and my two younger teenage siblings (who have autism, no smoking/drinking). I try to live by example (teetotaler, no smoking, active, vegan...)  but I don't see him often and he is surrounded by people who don't want to him, or themselves, to change. 

My older brother (also a heavy drinker and pot smoker) has also gone into a psychosis (not a figure of speech, he has rapidly gone downhill into a psychotic state) since finding out that my father has cancer and is now completely delusional about the world around him. My mother (teetotaler, non smoker) is devastated. My father doesn't even notice or care. My brother still lives with my mum, and I don't want to see her suffer.

In the past I have begged my father to quit smoking and drinking and offered him my full support to do so. However, my help is always rejected and I am always accused of being 'judgmental'. This is also ridiculous as I am constantly mocked and judged by my family for NOT smoking and drinking!

I told him a few months ago that it was only a matter of time until he got cancer, (or other disease caused by smoking and drinking), and that I had for years been rehearsing the conversation in my head in preparation for the news. I told him it wasn't too late and that I would support him every step of the way if he wanted to quit. This had no affect on him whatsoever. He actually laughed.

My father is in total denial about his cancer and it's likely causes. He refuses to quit smoking or drinking. I am the only sober, rational voice left in this family and I am trying to help, but I'm being made to feel as though I am an awful person. (I was literally shouted at, insulted and mocked by my father and his wife when I went to talk to my father about his cancer diagnoses this week.) My father is convincing my brother I am to blame and that I am a negative influence, and my brother is so delusional in his psychosis he is believing him.

Am I wrong to think that encouraging my father to quit and setting a good example is positive?
Am I wrong to think that smoking and drinking around my father is being negative?
Am I wrong to love him so much I don't want him to die prematurely of a preventable disease?

I tried to explain to my dad that I do believe he can quit, however I don't think he will. I don't for a moment doubt his capability to quit, I just doubt that he will ever want to do it enough to see it though. I hope he will prove me wrong.
I have told him that I cannot watch him smoke and drink himself into an premature death of a preventable disease. I love him too much to watch him kill himself. I love him too much to sit around and watch my brothers pass him cigarettes. I love him too much to see my brother sell him marajuana, then smoke it with him. I love him too much to see his wife pass him another beer.

This is breaking my heart. I would cut them [dad, his wife, step family] all out of my life only my mother (not my fathers wife!) is desperately upset that my brother (her son) has spiraled into psychosis.
I feel as if I'm being selfish, but I feel that their [my father and his wife] lifestyle choices are selfish, and they are making it impossible for me to be a part of their lives.
Every time I see them they get drunk and insult me. When they came over to see me in December my boyfriend was horrified that they turned up, drank a case of beer and spent two hours slagging me off while proclaiming they don't drink. This is what happens every time I see them. I just hoped that getting cancer would be the wake up call my dad needed to change.
Am I just wishing for something that will never come? Should I just stop kidding myself that I can ever have a relationship with my father and stop trying?

I would forgive everything that has happened between us up til now if he would just stop smoking and drinking.
Advice from smokers and alcoholics appreciated. I promise I'm not judgmental, I'm just frustrated.