My sister-in-law-to-be asked me and both of my sisters to be bridesmaids, and picked out dresses for us to buy that is lace and tulle (not stretchy). It does come in a maternity version for me, though it's not clear Its available in long (important since I'm 5'11"). However, she's also being very understanding, saying that she will not take offense if I don't buy the dress (which is $180 plus the cost of tax and shipping), can't stand up with the other bridesmaids during the ceremony, or even if I'm unable to attend. 

The attendance we will play by ear (I really want to be there if at all possible) and I'm helping plan the bachelorette even though I can't attend, but what would you do about the dress? Buy it out of loyalty even if it might turn out to be a waste of a couple hundred dollars? Buy a more affordable dress in the same color (and maybe a stretch material that's more likely to fit)? Thank her for her understanding and let her know I'll be there in whatever dress I own that fits?