Suggest an alternate, closer, less expensive location or bow out.
Every other year we have to vacation together for a reunion.
Why do you
have to vacation with her? Deathbed promise to a parent? Does she have the deed to your house? Can she fire you if you refuse? Still trying understand the notion of an adult having to do something when the repercussion something along the lines of disappointment, whining, anger, or hurt feelings all that should pass unless the person is an immature idiot in which case their company is not welcome regardless of shared name and/or DNA.