While there is no indication in anything I can find that the polyps are a "prelude" to oral cancer, (she didn't specify where in the throat the polyps were?), all the literature seems to point to the fact that they are caused by the same irritants that can lead to oral cancer. These irritants include tobacco and alcohol use. I think you have good reason to be concerned that your sister still smokes and drinks despite her surgery. But in all honesty, it seems that you are more concerned than your sister is!!!
Though you might encourage her to change her habits to ease your own mind, only she can stop her smoking and alcohol intake.
I am glad you found your cancer early! Hang in there, and keep us posted!

Live Well,
Andrea