I'm going to ruin my brother's vacation
Why would you want to ruin his vacation?
He ruined one of my earlier vacations
I'm sorry to hear he did that to you. Ruining your vacation was wrong, but it's just as bad to do it back to them! Being the bigger man would be much more positive.
What if he does it again? He's got cash and he's dating a very unpleasant woman who always puts her needs ahead of everyone else.
Sometimes you have to find ways to work around unpleasant relatives at family gatherings. Getting revenge will only exacerbate the situation and then everyone's vacation will be ruined. Plan ahead to minimize your contact with her.