I don't know how I (21F) can watch my brother (26M) slowly kill himself by smoking cigarettes.

My brother has been smoking cigarettes on and off since he was 15. He quit once or twice, but stress always causes him to go back to it.

Both of my grandmothers passed away fairly recently, ultimately from smoking cigarettes for most of their lives. It was horrible to watch, and I thought that would be enough to convince him to quit.

This is absolutely not the case. If anything, he's been smoking more. Today, I heard him in his room next door coughing. It sounded like he was hacking up a lung. I am so terrified that he is going to die a premature death because of this terrible habit.

I completely understand that the addiction to nicotine is so strong. I completely understand that hounding him about quitting won't do anything. I've cried to him about my fears of him dying because of this, but that did nothing either.

Does anyone have any advice as to how to help him realize he has to quit? Or, if not, advice as to how I can stop worrying about him constantly?

TL;DR: My brother smokes cigarettes and I am having trouble dealing with this, especially since our grandmothers passed away recently due to smoking for almost their whole lives.