Behold, Here's Poison by Georgette Heyer

When Gregory Matthews, patriarch of the Poplars is found dead one morning, imperious Aunt Harriet blames it on the roast duck he ate for supper. After all, she had warned him about his blood pressure. But a post-mortem determines the cause of death as murder by poison. Suspicion falls immediately amongst his bitter, quarrelsome family. Each has a motive; each, opportunity.

It falls to the quietly resourceful Inspector Hannasyde to sift through all the secrets and lies and discover just who killed Gregory Matthews. He faces the deadliest test of his career when members of the wealthy Matthews clan begin to die, one by one. With motives everywhere, it is no easy case for the inspector to solve in time to spare the next victim. Meet the Matthews -- while they last... Gregory, Harriet, Gertrude, Zoe, Guy, Stella and Randall.