This sounds very much like Ngaio Marsh's book Surfeit of Lampreys, titled Death of a Peer in the United States.
From Wikipedia:
Lord Wutherwood storms out of the Lampreys' top-floor flat and into the lift. He screams for his wife, Violet, to join him and one of the Lamprey twins accompanies Aunt Violet to the lift. Moments later, Violet begins screaming. Gabriel has been skewered through the eyeball and dies a short while later.
And
The Lampreys argue about the twins, Colin and Stephen, because both twins admit to being the one to accompany Aunt Violet to the lift.