Dropping a person won't affect your cabin. It's possible to book a Deluxe Family cabin with just 2 people (and probably with just 1, though I haven't tried). If you still want that cabin with 3 people, it won't be a problem.
The only possible exception is if the cabin was booked during the early booking period and the person dropping out was the lead guest and the only person eligible to book during that period. Even then, I'm not sure you'd lose the cabin.