I don't think anyone is saying that they have the POWER to dictate how family members spend their money. An honest conversation about wedding expenses right up front can resolve a lot of issues brought about by agreeing to be in someones wedding. I also don't think anyone thinks that a person is OBLIGATED to be in a wedding simply because they are asked. A lot of us just feel that family should come first, and being asked to participate in a wedding is a special honor that should be taken seriously. Putting off a vacation for one year so that you can stand up in support of a loved one making a major life commitment is the kid of selflessness that I think is lacking in a lot of familes these days. This attitude of putting your wants (not NEEDS, but wants) first is, I'll just say it (though I've been trying to avoid the word until now), selfish.
Your DH is close to his sisters, and you yourself are fond enough of one of them to watch her child every day for nothing more than "hugs and kisses"? You are hardly random family memebrs (I think that title is reserved for the second cousin you haven't seen in 15 years yet decide to ask to join the wedding party!) You could have compromised...say have just your DH be in the weddings instead of all of you. Or maybe just your child. The only expense for the two of them would have been the outfit, since neither would have been expected to throw a shower or have their hair done. By doing that, you could have made both sisters happy, and then no one would even be batting an eye at your vacation.
You are, of course, going to do what's right for you. It's just that you seemed shocked and upset by the reaction of the older sister to your vacation and linked it to her wedding. Some of us just feel that you really shouldn't be that surprised, based on the decisions that you have made and how you made them. I agree with some of the others who feel that this kind of situation surrounding a family wedding can have lasting negative effects on family relations.
I STILL wish you good luck with all of this and hope that given everything, you can have a nice vacation.