"Alfred Hitchcock Presents"Conversation Over a Corpse (1956)

Mr. Brenner makes a down payment on a house. The house's owners, Cissie and Johanna Enright say that they don't want to sell. He threatens them with a lawsuit, but is poisoned. He collapses and the sisters discuss what they should do. Joanna wants to finish him off more poison while Cissie prefers a gun. Brenner revives and pitches on sister against the other. He gets the gun from Cissie and shoots Johanna. Cissie gives Brenner more poison whilst he phones the police, telling them that Johanna poisoned him and that he shot her. Finally, the poison kills him and the house is all hers.