I have to agree with some other posters - I think Princess would be your best bet. We have been on NCL & Princess just recently. Both were good experiences. I think the parents and sister would do well with this option, and you could do anytime dining and still dress up. That's what we like to do.
On NCL we didn't have any trouble making dinner reservations like I hear some say. I usually just called about 10am and made them for that night and the following night. (We usually eat at 8-8:30, so this could have been a factor) The surcharge restaurants were very good - esp liked LeBistro. But you couldn't do these truly "freestyle" as they needed a reservation. Went to the regular dining room twice - but not impressed.
So even tho your sister doesn't want to make plans, you will still need to. I did like the NY Deli part of their buffet - nice change. I was on the Dawn - so needed ressy in the Mexican and Italian venues also (Both good)
Since we eat later, on NCL it was difficult to get to their shows. The early was way too early and the late one was over before we got there. Princess has one that caters to the late diners. And there is always Skywalkers for a great dance treat!
Whatever you decide, I wish you a wonderful trip! If you decide on Princess, be sure and try one of their souffles at night for dessert - heavenly!!