The other girl was Tan Xiao's little sister, who was first seen piggybacking on TX in his flashback right after Zhu Jiu's vagueish remarks. She had long cascading hair, whereas Zheng Yi's is always in a ponytail. The whoosh-whoosh flash-overlay violin-playing scene was done rather poorly. They should've at least blurred/changed the background in shots of his sister to signal flashbacks.