Joining in!
My worst was a weekend trip with MIL. We were driving 5 hours back home for a BUDGET trip for my niece's first birthday party and had an extra seat in the car, and my SIL asked DH behind my back if we could bring MIL out with us. Of course DH said yes. Problem is, MIL is just about the most negative, mean, complaining person ever, and there is a standing rule in their family that she never pays for anything EVER when she is with one of her sons. So, everywhere we went, we bought. I'm talking appetizers, drinks, steaks, dessert, the works. She even treated herself to some "forgotten" toiletries and other essentials that she needed. I tried to curb the amount of times we stopped by feeding the kids before we left places, making them go to the bathroom before we left, etc, but that didn't work. She kept asking DH to stop for bathroom and eating breaks. When I told the kids they were not getting anything, she kept saying that she would buy it if we wouldn't because she didn't want to "stuff her face"(her words) in front of them alone. And of course DH wouldn't let her do that. Our budget trip ended up costing a couple extra hundred dollars.
Then, even though we were more than an hour closer to home than where MIL was staying, SIL refused to drive her out to us when it was time to go home because her teenage DD had a softball game, so we had to make the 2 hour round-trip to get her and get back to the original point before hitting the road to get home. We had to leave my niece's birthday party before it was over in order to do this, which was the whole point of going out there to begin with! And every other word out of MIL's mouth is a curse word, plus it's impossible to hold a conversation with her because she is either A. talking about people and things that kids should not be hearing about (like what relative of hers is in jail, whose the latest pregnancy, divorce, who is having "baby-daddy" problems, etc - trust me, we do NOT see much of DH's extended family!!!) or she is 2. too busy being offended by everything I say to have a conversation with her.
Never,ever again. We will be taking a 4-door sedan everywhere we go for the rest of our lives!