Why is Josephine especially careful about the way she tells her sister of Mr. Mallard's death?

Mrs.Mallard was known to have heart trouble.

Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death. It was her sister Josephine who told her, in broken sentences; veiled hints that revealed in half concealing.