Blood Relatives (1978)

Montreal: Late at night the teenage Patricia flees into a police department, covered all over with blood. She states together with her cousin she took shelter from rain in an entry way on their way home from a party, when an unknown man threatened them, forced her cousin to perform oral sex and then killed her. Patricia could barely escape. The police starts searching among the known sex criminals - but then Patricia changes her statement and states her brother Andrew, who had an affair with his cousin, was the murderer. Inspector Carella doesn't quite believe both the statements.