After the Funeral by Agatha Christie
In After the Funeral, everyone gathers at the estate for the funeral of Richard Abernethie, and imagine the surprise among the clan when dotty, arty Aunt Cora says, “But he was murdered, wasn’t he?” They all try to brush her comment under the rug, but Mr. Abernethie's elderly lawyer and friend, Mr. Entwhistle remains bothered, her remark nagging at him. He asks a few questions, but then Aunt Cora ends up dead too, killed with a hatchet in her bed. Entwhistle calls in Poirot, because clearly Cora wouldn't have been killed if it hadn't been for that remark.
Poirot shows up a little late in this way, but Mr. Entwhistle is a good character, so it works. We have the standard mystery characters of the era, including the family butler, the motherly wife, the gambler, the hypochondriac, the actress, the stockbroker of questionable values, most of whom are potential suspects. Almost all of the family gained from Abernethie's death.
I feel like it was a fair...