The objectionable patriarch truly is the most dangerous role in detective fiction. Gregory Matthews pops his clogs via poison at a time when almost every member of his family has reason to wish him ill. Might the culprit be his thrift-obsessed sister, Harriet? The dramatic Zoe, wife of his late brother, who goes out of her way to perform virtue; or maybe her children, Stella and Guy, who both have reasons to fear being severed from the inheritance. And who could leave out the…