A sister ship is a ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

 of the same class as, or of virtually identical design to, another ship. Such vessels share a near-identical hull and superstructure layout, similar displacement, and roughly comparable features and equipment. Often, sisters become more differentiated during their service as their equipment (in the case of military ships, their armament) are separately altered.

The most famous sister ships are the RMS Titanic, the RMS Olympic

RMS Olympic

RMS Olympic was the lead ship of the Olympic-class ocean liners built for the White Star Line, which also included Titanic and Britannic...

 and the HMHS Britannic

HMHS Britannic

HMHS Britannic was the third and largest of the White Star Line. She was the sister ship of and , and was intended to enter service as a transatlantic passenger liner. She was launched just before the start of the First World War and was laid up at her builders in Belfast for many months before...

. Other famous sister ships include the Cunard liners

Cunard Line

Cunard Line is a British-American owned shipping company based at Carnival House in Southampton, England and operated by Carnival UK. It has been a leading operator of passenger ships on the North Atlantic for over a century...

 RMS Lusitania

RMS Lusitania

RMS Lusitania was a British ocean liner designed by Leonard Peskett and built by John Brown and Company of Clydebank, Scotland. The ship entered passenger service with the Cunard Line on 26 August 1907 and continued on the line's heavily-traveled passenger service between Liverpool, England and New...

 and RMS Mauretania

RMS Mauretania (1906)

RMS Mauretania was an ocean liner designed by Leonard Peskett and built by Swan, Hunter & Wigham Richardson at Wallsend, Tyne and Wear for the British Cunard Line, and launched on 20 September 1906. At the time, she was the largest and fastest ship in the world. Mauretania became a favourite among...

. Royal Caribbean International

Royal Caribbean International

Royal Caribbean International is a Norwegian and American cruise line brand based in Miami, Florida and owned by Royal Caribbean Cruises Ltd.. With 42 ships in service under 5 different brands and one more under construction, it controls a 25.6% share of the world cruise market...

's Explorer of the Seas

Explorer of the Seas

Explorer of the Seas is one of five s from Royal Caribbean International. She can accommodate over 3,100 guests, including scientists making use of a built-in atmospheric and oceanographic laboratory operated by the University of Miami's Rosenstiel School of Marine and Atmospheric...

and Adventure of the Seas

Adventure of the Seas

MS Adventure of the Seas is the third Voyager-class cruise ship operated by Royal Caribbean International that cannot transit the Panama Canal....

are modern sister ships.

For instance, the U.S. warship

Warship

A warship is a ship that is built and primarily intended for combat. Warships are usually built in a completely different way from merchant ships. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuvrable than merchant ships...

s USS Iowa

USS Iowa (BB-61)

USS Iowa was the lead ship of her class of battleship and the fourth in the United States Navy to be named in honor of the 29th state...

, USS New Jersey

USS New Jersey (BB-62)

USS New Jersey , is an , and was the second ship of the United States Navy to be named in honor of the U.S. state of New Jersey. New Jersey earned more battle stars for combat actions than the other three completed Iowa-class battleships, and is the only U.S...

, USS Missouri

USS Missouri (BB-63)

|USS Missouri is a United States Navy Iowa-class battleship, and was the fourth ship of the U.S. Navy to be named in honor of the U.S. state of Missouri...

, and USS Wisconsin

USS Wisconsin (BB-64)

USS Wisconsin , "Wisky" or "WisKy", is an , the second ship of the United States Navy to be named in honor of the U.S. state of Wisconsin...

, are all sister ships, each being an Iowa-class battleship

Iowa class battleship

The Iowa-class battleships were a class of fast battleships ordered by the United States Navy in 1939 and 1940 to escort the Fast Carrier Task Forces which would operate in the Pacific Theater of World War II. Six were ordered during the course of World War II, but only four were completed in...

.

Half-sister refers to a ship of the same class, but with some significant differences. One example of half-sisters are the First World War-era British s where the first two ships had four 15 inches (38 cm) guns, but the last ship, , had two 18 inches (46 cm) guns instead. Another example are the American s of the Second World War that came in "long-hull" and "short-hull" versions. The Cunard ocean liners RMS Queen Mary

RMS Queen Mary

RMS Queen Mary is a retired ocean liner that sailed primarily in the North Atlantic Ocean from 1936 to 1967 for the Cunard Line...

and RMS Queen Elizabeth

RMS Queen Elizabeth

RMS Queen Elizabeth was an ocean liner operated by the Cunard Line. Plying with her running mate Queen Mary as a luxury liner between Southampton, UK and New York City, USA via Cherbourg, France, she was also contracted for over twenty years to carry the Royal Mail as the second half of the two...

are sometimes spoken of as half-sisters due to their similar overall construction and complementary nature, despite the different number of funnels, and the raked bow of the Queen Elizabeth.

The generally accepted commercial distinction of a sister ship are
Type: Identical main type (Bulk, Tank, RoRo, etc.)
DWT: +/- 10% on the DWT (So if the ship is 100,000 DWT, then 90,000 to 110,000 DWT)
Built: +/- 5 years (So if the ship is built in 2000, then built 1995-2005)
Builder: Identical shipbuilding company name (NOT the ship yard location or the Country of build)

The critical overriding criteria are the same hull design. For example the popular TESS-57 standard design built by Tsunishi Shipbuilding are built in Japan, China and the Philippines. All the ships of this design are classed as sister ships