Death of a hawker

Amsterdam is normally sedate but today there is a riot in Newmarket Square. Constables have blocked access to adjoining Straight Tree Ditch Road all day. When the body of the "King" of the local street market is found in a room in his house on that street, his head bashed in, there are only two suspects: his lovely sister or the upstairs boarder. Which one is the killer? Grijipstra and de Gier must discover the murderer's identity before another crime can be committed.