Complex sentence: The, sister to the and, never served on the liner trade for which she was built, instead entering war service as a hospital ship as soon as she was completed--she lasted a year before being sunk by a mine.
Split into simpler sentences: The, sister ship to the and, never served on the liner trade for which she was built. Instead, she entered war service as a hospital ship as soon as she was completed, and lasted a year before being sunk by a mine.