Try approaching them without words, show them your concern and worry for them in this life and the next with actions.
Both of my parents smoke, though they are not Muslim, they recently started a step towards quitting with vapor cigarettes. I think they are thinking about living to see grandchildren inshallah now that their oldest daughter is married (me).
I shared this story because it might be something that could potentially be a good motivator for your family.
Inshallah my reply will be of help to you.