Who killed her

Sun Shanqi (17 years old, gifted student) was killed on the night of February 29 in leap year, and his amputated thigh was abandoned by Japanese ukiyo-e stockings and found in the river. Her mother, Ruoyun (42, a sculptor), later learned from the police that the girl was actually the owner of a pornographic website, and that customers could pay her to undress. The perfect family suddenly broke down, and Ruoyun divorced her husband Allan; Ruoyun's temperament has changed drastically, and the relationship between her young son (Sun Shanwen, 17 years old) deteriorates rapidly, causing the other party to rebel and apply for a court order to follow his father. But the nightmare is not over yet, Ruoyun learns that on the night of the incident four years ago, her daughter left home angrily and it turned out to be related to Allan. Allan, in order to keep it a secret, destroys the evidence that the police can solve the case. Ruoyun, who originally believed that her daughter was killed by a serial killer, was instantly confused. Who is the murderer?