You can try several different approaches. First, research more about smoking and cancer (on the web or anywhere), what cigarettes compose of, and what it's harmful effects are. Then try to explain this over dinner one day by hinting about it and how bad it is (or you can use a recent event in the news or related to your life). For example, you can start out solemnly with "I heard today that my friend's aunt died of lung cancer, and she smoked for 3 years. My friend's really sad." You could even go into detail like "And you know, my friend was describing the condition her aunt was in, and she said that her aunt looked so pale and gruesome with yellow teeth and chalk-like skin that she was repulsed." That may be bending the truth a little, but it's worth it if your parents do quit smoking. You can also mention what cigarettes are made up of (and I know they put some disgusting stuff in them). Another way to do this is try to make them feel really guilty (I mean, they kinda should be by making you and your sister inhaling second-hand smoke and making you feel uncomfortable). I don't know if they smoke in front of you and/or in the house. If they do, then you can have your sister start coughing when your mom or dad puffs smoke in the same room or something (I start to cough when I smell cigarette smoke :P), and then act dramatic and say that you or your sister will end up dead in the hospital one day because of all the smoke. Secondhand smoke also kills quite frequently, so you can mention that too. Question if they really care about you and your sister....and tell them that it feels so horrible to deal with all the smoke everyday and the terrible fear that they (as in your parents) can die any moment. This could cause them to yell at you, but it's worth a try.
You can also try this: talk to your mom and dad separately one day, and ask how they acquired the habit, and why they smoke. This way you can find out other ways to kill their habit. Try to understand, be calm when talking to them, and sweetly tell them (especially your mom) that you really care about them and don't want to see them die. Quitting also means saving a lot of money.
And if you have any ideas yourself, try them out! Good luck with everything! :D
Oh and the user Jenny who gave the link to that youtube video; it's really good, watch it and show your parents! :D