Why talk to your mom? Talk to your sister. You'r both adults--even if you think she doesn't necessarily behave like one. If you have an issue with her decision to hold a destination wedding, she's the person to talk to, and the only person to talk to. It's not your job to convince your parents that your sister's decision is a poor one. They're adults, too, and they can come to that conclusion on their own. If you decide not to go the wedding, obviously you'll have to tell your parents why, but don't use that as an opportunity to bash your sister's decision. I just get the sense that there's some triangulation in your family dynamic based on what you've shared, and it's arguably not particularly healthy.