I'm sorry your trip is getting hijacked. I would be bummed about that, too. In fact, I'd be pretty uptight about it myself. We've had things like that happen before too, and it was a less than stellar time (although it usually happens with a friend's family, as opposed to the in-laws).
I've also done DLR with the in-laws. Although, they didn't invite themselves...I actually invited them. And willingly paid for it all. And actually, I planned that trip specifically FOR them....lol, so I know it's not really the same, except that I *have*, at times, been in a position where I felt the in-laws expected us to pay for things that should be their responsibility. The
Disneyland trip though, that was a gift that I really *wanted* to give.
Others have given you great suggestions! Additionally, if it were me, I would *also* MAKE SURE that the in-laws COMPLETELY understand that you have a set budget for this trip, and that you will not have any extra cash for "last minute" extra expenditures (ie: paying for their meals, etc.). Make it a money thing. Just say, "This is a really expensive trip for us, too...so, we don't have extra cash to pay for things like airline tickets, etc. for others".
I know it can be veerrrrrry difficult to put your foot down with people who would be so bold as to suggest you purchase their airline tickets (seriously, what is THAT about?!!

). But believe me, after spending most of my life NOT putting my foot down in situations like that...I finally discovered that the few moments of anxiety connected to actually doing it is SOOOOOO much LESS stressful than the days/weeks of feeling like this ---->

. The earlier in life you learn that, the better off you are going to be.
And seriously....MEMORIZE the exact wording Malcon10's daughter used when politely, but firmly, letting their family members know what they were going to do. It's perfect! And then every time you feel like things are getting "weird" at the park, you can simply use that phrase with your own plans substituted for "space mt".
I hope you all manage to have a really good time! (And if it gets too awful...you can always toss them out of the boat in Pirates!

) Ok, not really.