Father: "That's great son. Who is it?" Son: "It's Sandra, the neighbor's daughter." Father: "Oh, I wish you hadn't said that. I have to tell you something son, but you must promise me not to tell your mom Sandra is actually your sister."
The boy is naturally bummed out, but life goes on. A couple of months later...
Son: "Daddy, I fell in love again and she is even hotter!" Father: "That's great son. Who is it?" Son: "It's Angela, The other neighbor's daughter." Father: "Oh, I wish you hadn't said that. Angela is also your" sister.
This went on...