Your sister's death was caused by failure of the heart, sir, due to total shock. Literally, she died of fright.
9 seconds sound clip from the The Pit and the Pendulum (1961) movie soundboard.
You can hear this line at 00:16:33 in the Blu-ray version of the movie.
Quote context
[...]
- I want to hear it from you, Doctor.
- Her death was inevitable, I fear, under the circumstances. This castle, this odious atmosphere.
- What has the castle to do with it?
- It's too late now, Charles. I...I didn't want him to know.
- I thought to spare you, sir.
[...]