A sister ship is a ship 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.
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 Courageous-classbattlecruisers where the first two ships had four 15-inch (381 mm) guns, but the last ship, HMS Furious, had two 18-inch (457 mm) guns instead. Another example are the American Essex-classaircraft carriers of the Second World War that came in "long-hull" and "short-hull" versions.
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.