Usually Sandals will offer an excpetion to wedding parties. Typically they will allow single occupancy for a small percentage of your guests.
This can sometimes work out to 1-2 people. If you were sure that your soon-to-be mother-in-law will be the only single, you may still be able to have your DW at a Sandals resort!
I would suggest talking to your Travel Agent and asking them to arrange this, because it is not guaranteed all the time. We have managed to work out this situation with some of our brides through our Sandals Rep. I would ask your TA to talk to their Rep and see what can be done before you cross Sandals off enitrely.
Also, in the case that they won't budge for you on this, there is always Beaches Resorts. Beaches is the family version of Sandals, so mostly everything that you love about Sandals will be included in a vacation at a Beaches resort.
Hope this has helped, and Good Luck!!
Blue Petal Destination Weddings