since you'd be making a change to their reservation.
however, I think the scenario mentioned above could work. You could change her to their stateroom as a name change. You might get some sort of fee for moving her out of your stateroom and a name change fee on the other reservation.
Then later once they make up their minds regarding the concierge stateroom at least partially coming, you can make the necessary changes then. Disney doesn't like lead guest changes, but they will do so as needed. Especially since they will most likely get 50% or more of the vacation price anyway. If they have insurance through
DCL, the other family may be able to put the $$ toward future cruise.
With it being concierge you have some different rules. And you'd have to talk with DCL to find out what they can do. But it's hard to do that without the other sibling's buy-in.
I seriously doubt that the stareroom will go unused at the port. Once cancelation is made or the family doesn't show, DCL can sell the cabin to someone else.
Divorce can be really hard for everyone involved. So sorry you're having to deal with this and worried about your vacation too. Love my MIL, but couldn't survive for 1 night in a stateroom on a ship with her...even a suite!