Death of a hawker - Janwillem van de Wetering

A riot has erupted in New Market Square, transforming the normally sedate streets of Amsterdam into a mass of angry protesters. So when the body of the "King" of the local street market is found in his house with his head bashed in, the police are puzzled. The adjoining street has been closed off all day, and the constables stationed outside the scene of the crime didn't detect any unusual activity. There are only two people who could have reasonably committed the violent act: the victim's roommate upstairs and his beautiful sister downstairs. Both claim to have seen or heard nothing suspicious when the crime took place. But something isn't adding up. Is one of them the killer?