We bought into Disney Vacation Club specifically so we could put up everyone in our VR (12 guest plus the 5 of us). I figured out that the cost of renting a week of a 3 bedroom villa and a 2 bedroom villa was half the cost of buying the points needed to cover it. So sponsoring one more whole-family trip in the next 45 years will make the DVC pay for itself. Although we've never been to WDW, everyone seems to rave about it, so we figured it would be a good investment.
We're sponsoring our entire family with accommodations and meal plan during our stay. None of our siblings nor my parents would be able to afford the kind of trip we're planning on their own (my inlaws could afford it, but I thought it would just be easier logistically to pay for it all ourselves). All the family members are responsible for their own flights down and park tickets, which is within their vacation budgets. Since they might all have different ideas of how much time they want to spend at the parks, I didn't want to dole out the money for 7 day park hoppers that would largely go unused for their intended purpose. We could realistically afford to pay for everything, but I didn't want to make my loved ones feel like they've lost all their own autonomy with the situation, or that we're lording our money over them. I'm a pretty frugal gal, so I put it in a way that we're just doing it in the most frugal manner to get the biggest bang for our buck.
Whether you are paying for your guests totally depends on the financial situation you are in, the financial situation they are in, and how close they are to you. There is absolutely no set rule, at least in my opinion. However, if you can't afford to sponsor them, and they can't afford to go on their own, there should be no hard feelings between you two.
Money is always a tricky situation to talk about. Do you know the financial situations of all of your guests? Is it something you could seek advice from your parents on how to approach with your guests? WDW is out and out an expensive proposition for the vast majority of Americans, so it would be a good idea to find out how your guests feel about it. Our best friends' friend (does that make sense?) is getting married in Hawaii, and many of the guests are resentful for the financial imposition that is being asked of them. But I think if it's something that the bride and groom really want for themselves, then they should go ahead and do it, but not have any expectations from their guests when it comes to attendance.
Good luck!
Jil