[retired]

Father: "That's great son. Who is she?"
Son: "It's Tina, 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.Tina 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 Penny, the other neighbor's daughter."
Father: "Ohhhh I wish you hadn't said that. Penny 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 daddy is their father!"
The mother hugs him affectionately and says:
"My love, you can date whoever you want. Don't listen to him. He is not your Father."!!!