As others have said, although Disney's policy is that everyone in a room needs to be on the same dining plan for the full duration of the reservation, there is a loophole created by the overlap of the ability to add guests at check-in and the rule that the dining plan must be added at least 48 hours prior to check-in. Just be warned, though, that whenever you're in a gray area, you're subject to the interpretation of the specific cast member you get at the front desk. Your experience may be different from someone else's.
What I really want to comment on, though, is your overall plan. As someone who frequently stays in
DVC studios, I have to ask if you've ever stayed in a DVC studio before, and if you have an idea of the size and layout?
Four people in a studio, with one of them being your mother-in-law, doesn't sound like much fun to me.

Personally, we only stay in studios when it's just the two of us.
Technically, you're within the occupancy limit of the room. Even if you're a very close extended family, though, you're going to be very crowded and won't have any privacy. You'll have a queen-sized bed and a full-sized pull out couch. When the couch is pulled out, there's not much room to move around. One person's bed time will pretty much have to be everyone's bed time.
For sanity sake, you may want to consider a larger room for the 3 days your MIL is with you.