Because you gave the reason of "too much money", it's easy (especially for the brides) to link these two seemingly unrelated events. Again, an honest discussion about the expense of both weddings at the time the second one was announced could have resulted in a compromise that would have worked for you financially, and made both sisters happy.
There are a lot of ways this could have worked out, but I think that the honest truth is that some people, you included, don't place a priority on family obligations. That's your right, and I get that every family is different, but it seems a little off to some of the rest of us who do place family first. If one of my sisters needs/wants me to be there for something, I'm there, and I know that they would do the same for me. Not all families work that way, I know. In your case, if the sister wasn't upset, we would never be having this discussion. However, she obviously is upset by your decision, and you have to live with how your actions make her feel.
This is less about money or cancelling a trip than about respecting the fact that your decision has hurt a family member. Stick by your plans, by all means, but try to be sensitive, not defensive, while you do.