BBB- I have heard alot of dr say -dont give up your addictions when preg because of the stress it can cause mum and baby. think its an indivual thing- if you think you can quit when preg then do if not...
If you are really stressed with quiting a Doctor MAY tell you that just reducing the level is better? I think maybe this is what the doctor meant?

However there is no safe level of smoking. Woman should still be encouraged to quit. Even reducing the amount of cigarettes smoked, still exposes the baby to harmful substances.
I would find it more believable if the doctor had said to your sister, "if you are unable to stop smoking completely - it is important to remember ‘the less you smoke, the less effects on the baby’. "