Brian's right that nicotine addiction is strong and the addicted person has to really want to quit for there to be a chance of success. I'm an ex-smoker and I certainly can attest to that.

This might seem like an odd suggestion, so ignore it if it strikes you badly, but al-anon has a ton of good material on how to emotionally handle living with and loving someone who has a destructive addiction. Of course, the material is talking about alcoholism as the destructive addiction, wich is entirely different in many ways, but the emotions you're describing experiencing and emotional drain from the the weekly broken promises, etc. seem pretty much the same. (I know this because I used to be married to an alcoholic).

Until your husband recognizes that he should quit smoking, you need to learn how to take care of your own emotions in this, especially given the history with your sister, and some of that material might help if you're open to it (I don't think they have a 12 step support group for people who live with smokers.They should!)

Its worth being aware too that one facet of addiction is that you as a loved one can get dragged in as a part of the dynamic that keeps it going. Your husband may never feel he really has to get decisive about trying to quit because you're right there telling him he needs to all the time. Just something to consider.

Nelie