She is your half sister!
One Sunday morning William burst into the living room and said, "Dad! Mom! I have some great news for you! I'm getting married to the most beautiful girl in town. She lives a block away and he
r name is Susan." After dinner, William's dad took him aside. "Son, I have to talk with you. Your mother and I have been married 30 years. She's a wonderful wife, but 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."
William was heartbroken. After eight months he eventually started dating again. A year later he came home and proudly announced, "Dianne said yes! We're getting married in June." Again, his father insisted on a private conversation and broke the sad news. "Dianne is your half-sister too, William. I'm awfully sorry about this."
William was furious. He finally decided to go to his mother with the news. "Dad has done so much harm. I guess I'm 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. "Don't pay any attention to what he says, dear. He's not really your father."