My boyfriend's dad is similar, if not worse. He is a chain-smoker...practically as soon as he's finished one, he'll light another. He unfortunately contracted MRSA which left him disabled, so he is not very mobile either...just sits in the same chair watching TV all day. He is also diabetic to top things off. He has recently seen a heart specialist who has told him that two of the main arteries/veins to the heart are completely blocked, and two are partially blocked. He also has many blood-clots in his legs and other parts of his body. They refuse to operate on him because of his health condition, and actually said to him that he would die on the operating table. But he still will not give up. His family, especially my boyfriend who is a non-smoker, have all tried to tell him that he won't live to see his six-year old son growing up, but he won't stop. The whole family have all had lung/chest infections, and they all have asthma. He does not help their conditions at all, and is determined not to quit. He even wears nicotine patches whilst smoking.
Hopefully your mum is not like this, but I guess her situatiojn is similar. Smoking is an addiction, and as she has smoked for a very long time, it will be extremely hard to give up. I can't give much advice, but I do know what you are going through. My boyfriend cries sometimes because he is scared of losing his dad, even though he hates him for causing ill health to him and the rest of the family. His dad is selfish, and that's the only way to describe him, but as for your mum, you may have to sit and talk with her about what the future holds, especially if you have younger siblings.