Your travel funds can be used by anyone, as long as the flight is booked before 1/28/2011. The actual travel can occur after that date. So, as long as you book the flight for your mother-in-law before 1/28/2011 AND your travel funds haven't expired, yes, you can use them.
From my most recent SW ticket
Effective January 28, 2011, unused travel funds may only be applied toward the purchase of future travel for the individual named on the ticket.