I might be the outlier here but TBH I'm not sure why the invite was extended anyways.
1) WDW or Disney in general isn't everyone's thing so you'd have to rely on these people, whom you don't have a strong connection with, to agree to go to a place that they need to likely be very interested in going to in the first place not to mention have the time to take off, the energy to do especially with young enough kids, the finances to afford all the other things (and tickets are so expensive just on their own never mind the food, the merch,
Genie+/ILL, etc).
2) So many people express here going with families is usually a herculean effort much less a portion of the family you don't have a close connection with.
3) from your standpoint you're disappointed at the lack of their interest/communication but I would have taken their lack of initial response for that extended period of time that they were not interested and likely wouldn't have prodded further. Last time you saw them was potentially up to 6 years ago depending on when in 2016. Coming at it from having estranged family members it wouldn't occur to me if trying to build back that bridge to do a grand vacation where it tries even the most patient people and can cost an incredible amount even without the lodging part being paid for.
I think your hearts were in the right place and I feel for your wife but baby steps would have been the way I would have gone. This sounds like it was a pipe dream where y'all were on cloud nine thinking of all the wonderful possibilities and now are upset when it didn't pan out the way you thought it would. I also don't necessarily view your brother-in-law in the most complete negative way, perhaps because of the family estrangements in my family I view it more practical here.
I see your last update and that would have been the advice I would have given which is plan
your trip. I hope you have a lovely time and the other family decides to come along it becomes that even better trip