WOW! Love your first line....very funny! I'm sure you know what your family will be discussing, at every family function, holiday...etc... from now until next year! Is this your ONLY sibling and is that why you accepted the invitation for your WHOLE family to participate in the wedding????? It's gonna cost you a pretty penny to have ALL OF YOU in the wedding party - I'm sure I don't have to tell you that.
Couple of thoughts here: Regarding your husband....it's a year away. Does he REALLY need to be part of the wedding party? (Makes more sense for you to be part of it sister/brother) just to save money.
Children and sizes: Don't buy a dress for your daughter and give actual sizes for your kids....noboby should expect you to do this a year in advance. Just think you are setting yourself up for failure. 3 - 4 months in advance, is much more reasonable. Also, the tuxedo rental place may offer a "free kid's tux" to the bride and groom (did that for my son who was a ring bearer.) I would definitely ask that question and see if that savings can be passed along to you - I think it may depend on how many tuxes are rented.
Bridesmaid's dresses: Consider having someone make them for a fraction of the price. I did this for my wedding way-back-when. Trust me, if a professional seamstress is making dresses, this person can have a world of influence on a bride when it comes to a "particular person" in the wedding party who they may not look flattering in a particular style, and the seamstress may be able to guide them to something else or alter a dress so that the material is the same-but style is slightly different. Alterations at a boutique or Bridal shop can be very expensive.
Down payments: If you have to go w/ a down payment for dresses, tuxes, etc......does it have to be the whole amount? Right before the holidays???? It really is a bit insensitive for her to expect a family to come up with so much money so early in the planning. And to book tuxes a year in advance? WHY? It's a rental store! Has your brother jumped in and given any help in resolving this???? It really sounds like a financial burden to your family (and you haven't even given a gift yet for the wedding or for any of the umpteen showers that you will attend or help plan!)
WOW! Really good luck. Hopefully when the novelty of the planning wears off a bit, she'll come to her senses.