Son:"Daddy, I fell in love & 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 u 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 ...

Son: "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 straight 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 whoever you want. Don't listen to him, he isn't your father."