I smoked in college, and I quit my senior year because I added up what I had spent on cigarettes for 4 years. And they weren't very expensive then! When I realized I could have gone on a unique and popular trip during winter break if I had just banked that money, I woke up. So totaling up what he has spent is not only a good exercise in math, it's pretty eye-opening. Are there things he wants for himself or his family that he cannot afford? What if cigarette money plus the added health costs from smoking damage were applied to something beneficial? Do some research and create a spread sheet or graph (good use of math and computer) that helps show the big picture. Work with your sister if she is equally concerned, and make a "professional" presentation that shows the impact and also, on the side, demonstrates that you can apply yourself to something that's important.
Does your dad smoke in the house? Are you and your sister exposed to this? Then you, as a family, need to present a united front on how this affects everyone. I had a friend die of lung cancer, and neither she nor anyone in her family smoked, nor did she work in a hazardous environment. Her last 7 years were sheer hell.
Encourage your mother to work with your father's doctor to develop a smoking cessation plan, using the patch or any of the other drugs available, and the types of support groups that help a serious addiction. This is a physical and emotional dependency that your has, and it has to be approached as a huge project, not just something his kids nag him about. And consider shutting off the TV and computers, and going for a family walk. He's probably not going to smoke while he walks, and if he builds up some good endorphins from exercise, maybe he won't be as quick to pick up a cigarette. Most smokers light up when their hands are relatively idle - watching TV, driving, sitting at a desk. Minimize those times, and get some family closeness as a bonus.