I feel very peculiar about this subject.
Smoking while pregnant is bad, we know that.
Every day there is something new which comes out saying what pregnant women should/should not do.
Today it's no caffeine (again).
So: no alcohol, no illegal drugs, no caffeine, no smoked foods, no unpasteurised foods, no shellfish, no uncooked meats or fish, no undercooked eggs etc
Then we go on to: no stress, no heavy lifting, no emotional difficulties, no depression or general mental unease
Do: exercise (but not too much or in the wrong way), take supplements (but not too much or in the wrong way), eat a good diet (see above many food omitted) and so on
I agree that it's better for women to lead a healthy life while pregnant. However I think that a lot of things are "banned" due to a risk which is actually very small.
Foetal alcohol syndrome requires a lot of drinking. Problems due to smoking require a lot of smoking. It would be very unfortunate to induce late miscarriage due to some dodgy prawns.
Most people know the advice on different subjects, and understand the risks. They make an evaluation which works for them.
I would hate to live in a society where people who didn't adhere to all the rules while pregnant were taken into protective custody. Which is the logical extension of this, I think.
Your sister smokes. Of course she knows she shouldn't. She probably feels guilty every time she lights up. Smoking is very addictive. Statistically, it is unlikely to do much damage to the baby. Give her a look when she does it, but otherwise leave her be.