Stated differently, the Marriott Rewards points are in an account in your MIL's name. That ownership of that account and the points in it are unrelated to the ownership of the timeshare.
Your MIL can accrue points in that account from hotel stays, Marriott Visa card charges and other earning opportunities. Points obtained in connection with the ownership of a Marriott timeshare is just another way of adding points to that account.
Thus, after she sells the timeshare, she will still have those points to use.