How can I help my parents quit smoking?

I’m 17 and my sister is 10. My parents are both almost 40, and they have been smoking since before I was born. We are so scared one of them is going to get cancer or something horrible. My dad wants to quit smoking, but my mom has never made an effort to quit except ordering the chantix pill which is now being connected to suicide. She says she wants to, but in my entire life the only times she has ever quit is when she was pregnant with my sister and I.

How can I help them? Any ideas? I would really appreciate it.