What's the difference in the deck plans between each of the ships? Weren't they sister-ships?

Indeed the base of Olympic and Titanic (and to some degree Britannic) were iron plans used by Harland and Wolff, which were identical. Still, when modifications were made, it was because design improvements were discovered by trial and error. As Olympic came first, any mistakes and corrections needed were implemented into Titanic — if they found time to do so. These included eliminating the large B Deck promenade to extend the First Class staterooms, enlarging the reception room and restaurant, and other factors to make the ship more financially viable. But it goes further than that. Tiny quality-of-life changes were implemented into Titanic before her maiden voyage thanks to the time spent and recommendations on Olympic. Most of these changes eventually found their way to Olympic when she entered a refit after Titanic's loss.

But Olympic at her maiden voyage was a completely different ship than Titanic when she began her maiden voyage. The Victualling department's working areas were wholly distinct. Ventilation needed to be more utilized and more significant. And the Turkish Baths were overdone and not as well designed as imagined.

Britannic was designed to fix all the problems and epitomize the Olympic-class. She was to be the flawless combination of passenger amenities, comfort, and safety. The knowledge and experience of two earlier sister ships were utilized to perfect the design. While the outside hull and shape mainly stayed the same, many of her interiors were re-organized and configured — a brand new ship with different rooms, new styles, and surprises.

If only the war never took her.