warning: wall of text. Lots of backstory. 

Several years ago my wife and I had a destination wedding in another state. Her whole family went. Her parents are divorced and the older of the two brothers lived with his mom.(this is the brother in question) Because they didn't have a lot of money they road with us in the car and took a bus home at the end of the week. (We stayed a second week by ourselves for a honeymoon). 

Her mom talks about that trip all the time. she said it was one of the best trip she ever took in her whole life. she talks about how much she loved to go back there someday. 

so this year for Christmas we decided to surprise her with a suitcase and a letter saying basically that we were taking her back there this summer. Because my wife sister is older and married now they will be going also. 

A few days ago I find out that the brother somehow thinks that he was invited to and that he's going on vacation with us. My wife doesn't really care so long as he pays his own way. 

Here's the problem though. This brother is like the most socially inept person I've ever met. He never learn to be an adult and is just a complete embarrassment to be around in public. He has this really annoying need for constant attention. If people are talking about things that he doesn't know about or have any experience with, he will just start making stuff up. He makes up insanely unbelievable stories about things he's done or seen that anybody would know is not true. And when you're talkin about things he will try to correct you and tell you you're wrong and then make up lies to support it. 

Ex: someone asked if Dr pepper is made by Coke or Pepsi. I explained the doctor pepper is its own company and not made by either. He chimes in that dr. Pepper is actually made by Coke but Coke keeps it a secret. I told him that's ridiculous. He tells me that his friend's dad is the CFO for Coca-Cola and told him that they make Dr pepper. 

It causes a lot of stress in the group. He will start trying to cut me down and tell lies. I will call him on his lies and expose him. Then he will get mad and pout and everyone will get mad at me for being mean to him. 

so anyway I find out that he thinks he's going on this vacation. I told my wife no way. If he's going I'm not going. We have going on a week-long trip with him before and it was a nightmare. everyone was mad at me the whole time for arguing with him. They think I should just let it go and let him lie. 

Part of me realizes that since I'm the only one that has a problem with him, I should bow out. But my wife and I are the ones that planned this trip. This is our vacation spot that we go to every year on our anniversary. (Forgot to mention this trip would be on our anniversary). We have gone every year since we were married and I really don't want to have to skip it.