I have my own, less harmful, addictions, and if I'm going through a challenging or painful event, I personally can't always say no. Example: I'm 10 years diabetic and controlling my blood sugar through very strict dieting, but sometimes I must have the comfort of chocolate. Most of the time, I can keep myself from thinking about it. But I cannot envision having to live the rest of my life never enjoying chocolate again, and it's nowhere near as addictive as nicotine. Just realizing how powerful this relatively shallow addiction is, I can barely imagine the will power it would take to willingly eliminate one of the strongest addictions experienced by humans.
And your MIL has been "cornered" by circumstance into giving up her habit. To be truly successful at conquering an addiction, we have to choose it, and follow through from our own resources. Your MIL has not made that choice. She may not be able to.
My grandmother smoked for many years, tried many times to quit, finally succeeded, but told me many years later that she had to fight the cravings daily. The very hardest times for her were when she should smoke least – when she was feeling miserable from some illness. She managed to avoid smoking, but admitted that it was almost beyond her will, and the effort was really draining. She felt diminished and depressed during those times, and part of the depression, in her understanding, was that she couldn't even have this one "little" thing to comfort herself. If she had just a tad less willpower, she would have taken up smoking again on several occasions.
My grandmother has been dead for over 25 years now, but still has my deepest admiration for her tremendous strength, courage, and sheer will. We're not all put together that way – I don't seem to be, in spite of my wish that I were.
I also had a middle-aged friend who died of lung cancer. He could not stop smoking, even as he approached death. He wanted to, and beat himself up for his weakness, but he could not stop. He was an admirable person, loved by those near him, in spite of this particular weakness.
So I hope you won't let this weakness that your MIL is expressing color your whole picture of who she is. Apparently, your father finds something in her worth his love and support. You have every right to decide whether you can be there to support him by helping her. That's really up to you.
I suspect that there is almost certainly more to your MIL than the lazy, careless person you have described. I've yet to find a person, even someone I don't like, who is that one-dimensional.
It's even possible that her medical problems, which have altered her body chemistry, have changed her brain chemistry in ways that are beyond her control. We don't blame people with mental illness for their inability to make better choices. And while many addictions are freely chosen at the very beginning, the user seldom has had any idea about how powerful that addiction will become, the possible future consequences, or even the reasons she has chosen to dabble in the first place.
So, my suggestion to you is that you look clearly at YOUR needs, and follow them. You may discover that supporting your dad is one of your needs, perhaps to the point of learning to deal with your judgements of your MIL. I'm finding this to be true of my mother, who treated me and my sisters abysmally during our childhoods. Yet now that she's becoming physically and mentally weak, I find I have a need to help care for her. Big surprise to me. But it's about who I am, not who she is. And I know I would, in the end, dislike myself if I were to turn my back on her now. So this is what life has handed me, and I'm surprised to find myself growing into the challenge.