I agree with HeatherN that this is one thing you don't want to surprise their whole family with. Most of the MAW families who have posted trip reports have said the same things as HeatherN - there were so many things to do and they were so busy that they felt they were not able to fit in all the things that were available to them. Even if you talk to your sister-in-law ahead of time, keep in mind that she really does not know what is in store for them and she could easily say it's OK, but find out once they are in the trip that it just won't work.
There are also lots of links in post #3 of the disABILITIES FAQs thread about MAW and other WISH trips. It includes a link to a WISH trip planning thread on the Community Board. You may want to ask your question on that WISH Planning thread since many people who have gone on WISH trips hang out there to give advice to people who will be going on trips. It's a sweet sentiment that you want to spend time with them on their trip, but don't be surprised if the past WISH Trippers give the same advice as HeatherN did.
The disABILITIES FAQs thread is near the top of the disABILITIES Board or you can use the link in my signature to get there directly.
Many kids on MAW trips stay at Give Kids the World Village, but some stay elsewhere. It might be because there is not availability when they are going. When that happens, there is no specific resort that is used.
Some kids want to stay somewhere else as part of their wish, so that is why they are not staying at GKTW village.
The village has a lot of advantages - first, it is supported by volunteers and has a lot of things that are free to the family - a restaurant with free food, an ice cream shop. Second, the whole place is wheelchair accessible, so no one has to be concerned about being able to get everywhere. Third, the villas are like houses, much better for many people's needs than staying in just a resort room (no matter how nice the resort). The last point is that everyone staying there has a child (children) with a life threatening condition, so it is one place where the family is 'usual', not unusual. Some people have posted that was a very nice advantage.