Trackdown (movie, 1976)

When his sister Betsy packs up and leaves the family's Montana cattle ranch to find fame and fortune in Hollywood, her brother Jim decides to follow after her to make sure she doesn't get into trouble. He's a little too late, however, since almost as soon as she gets off the bus, Betsy has her belongings stolen, then gets kidnapped, gang-raped, and is sold to a pimp to work for him as a prostitute. It is now up to Jim, with help from social worker Lynn, to rescue his sister and set things right.