"Confession" joke

A man is on his deathbed, and his wife is sitting by his side.

The man says to the wife: "Hon, I have to tell you something."

The wife replies: "Yes, you can tell me anything."

Man: "I slept with your sister"

Wife: "I know"

Man: "And your mom"

Wife: "I know"

Man: "I also slept with your secretary, Mary"

The wife puts a finger to his mouth and says "I know, my darling, now just relax and let the poison work"