Secrets and Stories

I once, after the death of both my husband and mother in law, asked a sister in law what I thought was a rhetorical question.
"Oh, I can tell you that, Frances," she said, and proceeded to tell me a confidence that her mother had once shared with her.
I was shocked. By the information - not that it was so shocking, but because it was so out of character with the woman that I thought I knew - but that she would reveal what was obviously meant to be a secret. Does death negate this?
I didn't pass this on. However, the sister in law told her own daughter, who told all the cousins, etc etc.
My own daughter has said that, irrespective of the ethics, she is pleased to have this information. A valid point.
The urge to confide is both strong and puzzling. My mother in law had not told her other daughter, who sometimes seems disbelieving of the whole story.
Are the dead non-people whose wishes no longer need to be respected?
Perhaps they are. I don't think that I will share any information that I regard as among my secrets.