Chelsea

One Sunday morning, Chelsea burst into the living quarters at the White House and said, "Dad! Mom! I have some great news for you! I am getting married to the greatest hunk in Washington. He lives in Georgetown and his name is Matt."

After dinner, the President took Chelsea aside. "Honey, I have to talk with you. Your Mother and I have been married a long time. She's a wonderful wife but she's never offered much excitement in the bedroom, so I have fooled around with other women a lot. Matt is actually your half-brother, and I'm afraid you can't marry him."

Chelsea was heartbroken, but after eight months she eventually started dating again.

A year later she came home and very proudly announced, "Robert asked me to marry him! We're getting married in June." Again her father insisted on another private conversation and broke the sad news. "Robert is your half-brother too, Honey. I'm awfully sorry about this."

Chelsea was furious! She finally decided to go to her Mother and tell her.

"Dad has done so much harm. I guess I'm never going to get married," she complained. "Every time I fall in love, Dad tells me the guy is my half-brother."

Hillary just shook her head. Don't pay any attention to what he says dear. He's not really your father."

Thanks to Phreekbird for this joke :)