How about changing the FIL, MIL names on Room 2 to be the wife and daughter #1? You have to keep at least one original party's name on a booking. Daughter 1 is going to sail in the future, so that's the one original that remains. Then the credits for room #2 will be in the wife and the two daughters:
New setup:
Spacious Balcony Room #1 - Myself, Wife, Son, Daughter #1 Spacious Balcony Room #2 - Wife, Daughter #1, Daughter #2 Spacious Balcony Room #3 - Aunt-in-law
You will only be out the deposit for room #3 at that point. Then if you decide you cancel you will end up with the credits in your immediate family members' names only.
I know it sucks you can't do anything about that third one, but at least you may be able to shuffle around on the second room. And if you do sail, then you will have the 2 room already.
