It sounds like the little birdy may not be your best source of info, but since it has already been done....it's time to move on.
With that said, you already know that you have problems with this family dynamic. There is no getting around that. You are now aware of the potential of the one person wearing a specific dress. You can, however, INSIST on a certain color. It is YOUR wedding.
Many weddings are now being done with the bridesmaids wearing dresses of the same color but different styles (just get a swatch and give one to every person who must purchase a dress) and allow them all to purchase the one that is most flattering to their personal tastes, finances, and body size/shape. This can also include the flower girl. The groomsmen can accent the look by purchasing/renting matching ties, bow ties, vests, or whatever suits your taste. The bridesmaids' flowers and groomsmen's boutonnieres can also be used to unify the color choice, thus reducing the differences in the dresses. Because of the issues, I'd also insist on a specific color of nylons and shoes, but again, I'd let them decide on the style. If she looks silly, too bad!
You are correct about not involving anyone else in these issues. It will just make matters worse. If the "other" family does, then the problem is on them, not you. I would not, however, go on any other shopping trips with a person who does not appreciate your tastes and your preferences. You'll just be opening yourself up for more negative comments.
Judging from your post, it wouldn't be a serious problem if the person making all the fuss just didn't make it to the service. Again, the problem will be on them, not you.
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