according to the members guide book "reservation points per person are based on double occupancy and apply to 1st/2nd passenger".
it doesnt look good for your MIL, sorry.
you could always be a great DIL and pay the $75 dollars to change you ressies to 3 people and pay the $75 fee to make your MIL ressie and adding your fourth person to her stateroom. not a bad idea
when you get aboard your MIL can still have the room all to herself and your family can still be all together.
no one is the wiser, except for anyone reading this post