You book part of the stay with your membership (reservation #1) and your mom books part of the stay with her membership (reservation #2). You can each call MS if you like or book the two reservations online. Then, assuming you booked the same resort, room size and booking category for both reservations, one of you calls MS to link the two reservations so that the room assigner puts you in the same room for the entire stay.
As the PP states, if you are an Associate on your mom's membership then you can make reservations using her points. In that case you can make both reservations yourself on the phone with MS and then have MS link them.