How do you have a father-in-law if you're not married yet? Is this the father from your current marriage? I'm so confused!

Is your fiancé upset about their father taking it over and that's why he wants to just go to Rio? I mean, it's just an engagement party, it's not really that important in the end. If it was your actual wedding, I'd be making noise. Perhaps discuss with your companion about what you both want and work with the FIL. Then make sure you put your foot down when it comes to the actual wedding.

engagement parties ... i'd understand eloping if they tried to take over your wedding...

If the father-in-law is footing the bill, then let him plan it.

As someone who lives near Reno--go down to Virginia City and ask the people at St. Mary's in the Mountains if you can get married there. It's a gorgeous spot with an old church, and you can rub it in the jerks' faces that you had such a fun vacation\wedding\honeymoon without them. (Plus Lake Tahoe isn't that far away...you can just drive or catch a bus up there and REALLY rub it in.)