Second Sister by Chan Ho-Kei

Nga-Yee, a librarian, lives a quiet life with her fifteen-year-old sister Siu-Man. After a difficult, impoverished upbringing and the deaths of their parents, they are finally finding a bit of stability. Then one day, Nga-Yee comes home to find her teenage sister has jumped to her death. Was it suicide, or was she pushed? And does it have anything to do with a recent trip on the Hong Kong subway which left Siu-Man silent and withdrawn? Nga-Yee cannot rest until she knows the truth about her sister – even if that means tracking down her sister’s friends one by one and making them confess.