Son: "Daddy, I fell in love and want to date this awesome girl!"
Father: "That's great, son! Who is she?"
Son: "It's Sandra, the neighbour'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...
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 neighbour's daughter".
Father: "Ohhh, I wish you hadn't said that. Angela is also your sister".
This went on a few more times, and finally, 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 and I can't date any of them because dad is their father!"
The mother hugs him affectionately and says "You can date whoever you want.