Just tell them you won't.
I didn't go to my brothers "wedding." It was his second. I spent a lot of money and time going to his first (in Europe, where his first wife was from.)
For the second marriage, he went snowboardingin Tahoe with a woman he had been dating a few months, and they decided to go to Reno and get married. 7 months later they wanted a "real wedding" ceremony and reception. They live in California and I live in Virginia, I didn't fly out for it. They divorced three years later.