M.F.
If your mom can't get out on her own, maybe you and/or your sister could pick her up and bring her to one of your houses for a visit.
In general, if visits are on your own "turf" you'll have the upper hand, even if the step-dad is involved.
For example, if your step-dad makes a comment about not being included, you can politely tell him that you'd love for him to spend more time with the kids, but you are concerned that his habits and behavior are not healthy influences (and this should be said in a sugary, syrupy sweet way...). If he could control his tongue and put his smoking habit on hold for a while, he'd be more than welcome to come over with your mom...
Have you heard about "Parenting with Love & Logic"? It's a great book, but the nice thing is that "love & logic" tactics work well on most adults too. (Even stubborn old men...)