Ha ha.

Here's an oldie but goodie -

Jake was getting fed up. He lived in a small village and his father was well known for - how can I say this - putting it about. Every time he got interested in a girl, his father would take him on one side, and explain with embarrassment that he couldn't let the relationship develop, because the girl was his half-sister. This had happened several times, and Jake was getting really depressed. His mother had noticed his bad moods, and asked him what was wrong. Eventually he plucked up the courage to tell her. To his surprise, she burst out laughing. "Oh, don't worry about that" she said "you see, he's not your father".