I've been on two NCL cruises covering a total of 21 nights, including eight nights on the NCL Star to the Mexican Riviera in October 2005 (at that time it was eight nights and went as far south as Acapulco). I've also been on five Carnival cruises, including the Carnival Pride to the Mexican Riviera in May 2004.
First of all, I'm not sure what you mean by a mini suite on Carnival. You could be referring to a category 9A cabin or a category 11 cabin. Carnival doesn't have any cabins that it refers to as a mini suite. If you are going on the Carnival Pride, the category 9A cabins are bigger than the normal cabins (category 8 and under). However, that is only true for the Pride and the other three Spirit class ships. If you are going on the Carnival Splendor (starting on April 5), the category 9 cabins are the same size as the standard sized cabins (category 8 and under). The difference is a category 9 cabin is a rear corner cabin with a wrap-around balcony. On the Spirit class ship the rear corner cabins with wrap around balconies are category 11 cabins that only sleep two. NCL does have cabins that it refers to as mini suites.
Carnival has an alternative restaurant that your sister might be interested in. It cost an extra $30 per person. It is called the David Supper Club. I ate there one night. It was very good. The NCL Star does have many more restaurants, including the two main restaurants, an Italian restaurant, a Tex-Mex restaurant, an Oriental restaurant, a French restaurant, a steakhouse and a Benihana style restaurant.
To describe all the workings of NCL's Freestyle would take a while. I'll give you the short version. There are three kinds of restaurants:
1) Free - no reservations (the main restaurants)
2) Free - reservations recommended (the specialty restaurants)
3) Extra cost - reservations recommended
Sometimes the extra cost restaurants can be had for half price. If so, the time will be listed in the Freestyle Daily (the ship's newspaper). However, this will be during the slow time. When I was on the Star, that was the first hour the restaurants were open.
In my opinion the free food on Carnival is better than the free (main restaurants) food on NCL. It is about equal to the free (specialty restaurants) on NCL. The food in the David Supper Club is about the same quality as the food in the extra cost restaurants on NCL.
My experience in the main restaurant on the NCL Star was not good (not bad - just not good). I only eat there once. My first night I enjoyed the outdoor BBQ and one night I ate (for free) in the Italian restaurant (I don't know if it is still free). The BBQ chicken was great. The Italian restaurant was good. The other five nights I ate at extra cost restaurants. Yes, the food and service is better at the extra cost restaurants. Who is going to pay extra for the same quality food and service as you can get for free?
Otherwise, the two lines are very similar. The Pride has a water slide, the Star as two (right next to each other). The Star has a better kids pool than the Pride. However, the Star's main pools are not heated and cannot be covered. The Pride has at least one heated pool (the one we were in) and has one main pool that can be covered.
If I had a family like yours (two adults and two children) I would choose the Pride. There are two reasons. One, in my opinion, the overall quality of Carnival exceeds that of NCL. Two, with a family of four, the extra cost restaurants are going to add up fast (if you choose to eat there). On the Pride we let the kids eat early and sent them to Camp Carnival. Then the three adults had dinner in the David Supper Club one night. Of course you can do the same thing on the Star, however, I prefer the set dinner times of Carnival over the Freestyle meal service of NCL. So, you can either pay extra for every meal or you can deal with the Freestyle method of dining. As I said, I prefer the set dining times.
I didn't write a review for my cruise on the Pride. However, I will say that it is currently one of my two favorite cruises (I have been on twelve). I did write a review for my cruise on the Star. It is so long I had to split it into two parts. If you want to read it, click on the links below ...
Part one -
http://www.cruise-addicts.com/reviews/readreview.php?id=176
Part two -
http://www.cruise-addicts.com/reviews/readreview.php?id=177