One Sunday morning Joe burst into the living room and said: "Dad, Mom, I have some great news for you! I am getting married to the most beautiful girl in town. She lives a block away and her name is Susan."
After dinner, Joe's dad took him aside and said: "Son, I have to talk with you. Your mother and I have been married 30 years. She's a wonderful wife but she has never offered much excitement in the bedroom, so I used to fool around with women a lot. Susan is actually your half-sister, and I'm afraid you can't marry her."
Joe was heartbroken. After eight months he eventually started dating girls again. A year later he came home and very proudly announced: "Dianne said yes! We are getting married in June."
Again his father insisted on another private conversation and broke the sad news: "Diane is your half-sister too, Joe, I am very sorry about this."
Joe was furious. He finally decided to go to his mother with the news.
"Dad has done so much harm. I guess I am never going to get married," he complained. "Every time I fall in love, Dad tells me the girl is my half-sister."
His mother just shook her head and said: "Don't pay any attention to what he says, dear. He's not really your father."