Complex sentence: She was one of three ships in her class: her sister ships "Petropavlovsk" and "Sevastopol" were both sunk during the Russo-Japanese War, while she survived it.
Split into simpler sentences: She was one of three ships in her class. Her sister ships "Petropavlovsk" and "Sevastopol" were both sunk during the Russo-Japanese War, while she survived it.