Crooked House by Agatha Christie

The Leonides are one big happy family living in a sprawling, ramshackle mansion. That is until the head of the household, Aristide, is murdered with a fatal barbiturate injection.

Suspicion naturally falls on the old man’s young widow, fifty years his junior. But the murderer has reckoned without the tenacity of Charles Hayward, fiancé of the late millionaire’s granddaughter.