Actually... I think there is now a way you can use your timeshare ownership in order to stay at a Wyndham Hotel property, such as the Grand. I don't recommend using it however.
Wyndham now allows you to "convert" your Timeshare ownership points into their hotel rewards program, "Wyndham Rewards", points. Wyndham Rewards points can be used for free stays at participating Wyndham hotels.
The Value however of doing so is not there at all as it takes an extremely large number of Timeshare points to earn enough Wyndham Rewards points to qualify for a free stay at the Grand. You basically would spend easily 2 or 3 times the points for a week in a nice hotel room as you would need to for a week in a 1Bdrm Presidential unit at the Timeshare resort in the tower next door.
The practicality, actual costs, or even the details on how to do it however are often not even mentioned or hinted at when the timeshare Sales people are telling you about all the benefits of ownership. "Not only can you use your points for stays at any Wyndham Vacation Resort property, but you can also redeem your points for free rental cars, airfare, or hotel stays at places like the Wyndham Grand!" (A rental car would cost easily over $1000 for a week if you calculate your cost-per-point on the number of points needed for the car rental)
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This is also a general FYI on the way things work so that you can let your Brother and SIL know and they'll be aware of it. They may also want to check out the Wyndham Owners Group forums to learn how to best utilize the points they have purchased.