There are a couple episodes where stewie smokes, but only one where Peter and Louis know about it, and I believe at the end he quits, and Brian starts back up (in the episode where Peter becomes president of the El Darado Cigarettes company), but for most episodes, Brian is the smoker (and drinker), not Stewie

Now in the beggining of the episode where Peter goes out on a fishing trip with his friends, and ends up being washed up on an island and Brian ends up marrying Louis, Peter comes back home and Louis asks "did you catch any fish?", and Peter said "No, but I cought this turtle, and named it (forgot what he named the turtle), then ate it, and saved the shell as an ashtray for Stewie"

Wait a minute... An ashtray for Stewie? Doesn't he mean Brian? The smoker of the family?