Destination wedding or not, weddings are an expense for everyone involved. While I understand where you and the OP are coming from, the truth is she was wrong when it comes to etiquette. Of course, she can choose not to follow etiquette. However, etiquette dictates that she not split up families in the manner that she has. And the reason why etiquette dictates that is so brides such as herself could avoid the headache she has with these friends. The problem is most people do not know etiquette since it is not taught the way it once was back in the day. Again, herself and other brides planning weddings would do well to pick up an Emily Post or Miss Manner's etiquette book to guide through the planning as not to unintentionally ruffle any feathers.
However, OP's exclusion of the other siblings gave the friend no right to tell her off like the way she did. That was wrong and uncalled for.