One Sunday morning Michael 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.
Michael was brokenhearted. He broke up with Susan the next day.
After eight months he eventually started dating girls again. A year later he came home and very proudly announced, "Diane said yes! We're getting married in June." Again his father insisted on another private conversation and broke the sad news. "Diane is your half sister too, Michael. I'm awfully sorry about this."
Michael was livid! He broke up with Diane that same day, leaving her in tears. He finally decided to go to his mother with the news his father had shared and tell her about his father's secret.
"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."
"Oh, " his mother shook her head, "What are you listening to him for? He's not even your real father."