The Mystery at Underwood House by Clara Benson

Old Philip Haynes was never happier than when his family were at each other's throats. Even after his death the terms of his will ensured they would keep on feuding. But now three people are dead and the accusations are flying. Can there really be a murderer in the family? Torn between friendship and duty, Angela Marchmont must find out the truth before the killer can strike again.
The Mystery at Underwood House is the latest exciting 1920s whodunit featuring reluctant ‘lady detective’ Angela Marchmont.