As of January, 2009 DVC Resorts are available thru RCI. Recently there have been DVC villas available within about 10 months. DVC members cannot make reservations at their own resorts until 11 months at the earliest, so RCI reservations are not likely to be available any earlier than that either.
Availability will depend on the trading strength of your in-laws resort as well as the dates and type of villa they would want at DVC. As with any other exchange, the more flexible regarding dates, resort and room-size - the more likely you will find success. With RCI, I'd suggest an ongoing search even a couple of years in advance of when you'd want to go.
Good luck!