In Dorian's introduction he says that body parts are taken away by the coroner, although they might still find fragments of bone or soft tissue. At the paint factory, they find several identifiable body parts, including hands and arms.
When Sarah tells her mother about her new job, her mother responds with an anecdote about Sarah sleeping in her parents' bed from age four until she was twelve - "that's why your brother is eight years younger than you". If Sarah was the cause of a sex-free marriage during those years, her brother would be twelve years younger.