Complex sentence: At the start of World War I, "Roon" was serving as the flagship of the III Scouting Group, and her sister "Yorck" was brought out of the reserve fleet, also into the III SG.
Split into simpler sentences: At the start of World War I, "Roon" was serving as the flagship of the III Scouting Group. Her sister "Yorck" was brought out of the reserve fleet and joined her in the III SG.