Somewhere A Bird Is Singing by E. V. Thompson
Sally is an orphan living in Plymouth's Barbican, a late 19th century dockside slum, where a vile trade flourishes in young girls, duped and shipped off to the brothels of Europe. Her sister, Ruth, is driven to prostitution in order to support them, until she becomes too ill to work.
With her sick sister to take care of in their single room home, Sally finds employment delivering wares for a local shop, meeting fisherman Ethan and, through him, Eva, a captain in the local Salvation Army. Eva has dedicated her life to rescuing the endangered girls and bringing their procurers to justice.
When Ruth is brutally murdered, Sally finds the father she has never known, but he brings only trouble and unhappiness into her life. When Ethan, his father and brothers are reported missing at sea on a fishing ship, Sally volunteers to help Eva's crusade - with near disastrous results.