Found in the Comments section of a Gateway Pundit story:

Son: “Dad, I’ve fallen in love and want to date this awesome girl!”
Father: “That’s great son. Who is she ?”
Son: “It’s Sandra, the neighbor’s daughter” ;
Father: “Ohhh I wish you hadn’t said that. I have to tell you something son, but you must promise not to tell your mother. Sandra is actually your sister. . . . .
The boy is naturally bummed out, but a couple of months later:”Daddy, I fell in love again and she is even hotter!”
Father: “That’s great son. Who is she ?”
Son: “It’s Angela, the other neighbor’s daughter.
Father: “Ohhhh I wish you hadn’t said that. Angela is also your sister.”
This went on couple of times and the son was so mad, he went to his mother crying.
Son : “Mum I am so mad at dad! I fell in love with six girls but I can’t date any of them because dad is their father!
The mother hugs him affectionately and says: “My love, you can date whomever you want. Don’t listen to him. He’s not your father”.