joko, i can only but agree with you here, you expect youngsters to encourage each other as it seems to be part of the ''smoking we look cool culture''.
but for a couple of 50 year olds that have had children etc, it defys belief, they are very narrow minded, and extremely selfish, a bigger concern i would have is, your sister going on holiday with these idiots, their not friends, and i suspect they will {as you,ve pointed out} encourage her to smoke/drink at every opportunity, and i CERTAINLY believe that they won't respect the fact that your sister is pregnant and SMOKE in front of her.
At least your sister has the commonsense, and can see the dangers smoking/drinking has on individuals, let alone being pregnant, your sister has to be strong and rise above these idiots, as its plainly obvious that no amount of explaining the dangers of smoking/drinking to her so-called mates will change their views on the said subjects, she should tell them straight to their face, that shes seriously thinking of not going away with them as she feels they are a bad influence on her and her babys health.
IF they can digest this and respect the fact that your sister dosen't want to smoke/drink, well all well and good, but i suspect they won't, its your sisters call at the end of the day, but if you don't ask you don't get, admitterly your sister should'nt have to sacrifice her holiday, but it seems the only logical choice.
Hope all works out fine for her, and good luck on her pregnancy.