Bix, Spruce or Le Colonial
I would parrot sfcitygirl and recommend skipping Le Colonial as overrated and overpriced to boot.
Spruce is new but tends to get almost universal praise. Unfortunately it is a fairly hard reservation to get.
If you can't get a table at Spruce then by all means go to Bix. It is a lovely restaurant with a great atmosphere. The food is very good and while the place is always bustling, Bix is somehow intimate and warm enough to permit good visiting during your meal. Not every highly-recommended place shares this quality.
I'll third these recommendations. Try to get reservations in the following order:
3. Anywhere but Le Colonial
Judging by your sisters' picks, you are looking for a swank and fun night out, right? I'd also suggest the lobby lounge at the St. Regis - yummy bites are made by Ame restaurant, and their cozy tables and chairs are great for people-watching or for an intimate conversation.