"You don't take a date to a wedding, that's like taking a deer carcass out on a hunting trip!" -Charlie Sheen
When I was a kid my grandmother told me a story about how Gramps missed her cousin's wedding because they scheduled it for pheasant opener. He had said if they chose that date he wouldn't be there, and he was true to his word. He's been gone 25 years and I still look up to him.
You gotta chart your own course on this one. My two brother in laws knew me well enough not to pull a stunt like that, but I dated their sister seven years before we got hitched. Yeah, I don't rush into anything!!
In my eyes, a destination wedding is a bit of a selfish gamble as far as attendance goes. Talk it through with your wife and see if she'd be okay going with her mom (and a healthy batch of fun money). Trying to use going/not going as a bargaining chip (for either of you) probably isn't in anyone's best interest. If she's super close with her brother, you're probably screwed either way.
Best of luck!!!