Two madmen were talking in the mental hospital, and one said, "I have decided to give my sister to you in marriage once we are out of here."
The other man said, "No, thank you. It cannot be so."
He asked, "Why?" The man answered, "Because, according to our family tradition only relatives get married. We cannot marry an outsider."
The other man asked, "How come there is such a family custom?"
He said, "You see, my grandmother married my grandfather. My mother married my father, my sister married my brother-in-law, and so on. How can I marry your sister?"