It depends on how the initial conversation went. If you said, "hey do you want to go on vacation with us? Your share would be 'x.' (which it sounds like you did,) then if they no longer want to pay, too bad.
If you invited, and then later said 'by the way you owe us 'x' that would be another thing altogether. I know it's hard when it's family, but personally, I would not want to go on vacation with someone like this at all! It sounds like they pouted when they weren't invited, and pouted when they weren't invited for free! Who wants to vacation with a pouter? It will always be something with a person like that!
Exactly! He sounds like quite a guy. Imagine how much "negotiating" will have go to on throughout the vacation - i.e. when you are out to dinner or at the ice cream stand, for example. Yikes.
We have paid for many a person to vacation with us. But the minute they were to ever to presume that or invite themselves or expect us to cover things that would change the game.
If I'm not planning to pay for someone but wouldn't mind spending some of the time with them then I just say "Hey, btw, we are headed to Disney again in May. These are our dates - we'd love to run into you at the parks!"