A guy went up to his father saying : Daddy , I fell in love and want to date this awesome girl !
Father : Thats great son , Who is it ?
...
Son : Its sandra , the neighbour's daughter
Father : Ohh ! I wish you hadnt said that .I have to tell you something son, but you must promise not to tell your mother. Sandra is actually yur sister
The boy naturally bummed out, but life goes on and indeed a couple of months later
Son: Daddy, I fell in love again and she is even hotter!
Father : Thats great son who is it ?
Son : Its angela , The other neighbours daughter
Father : Ohhh ! I wish you hadnt said that . Angela is also yur sister This went on couple of times and the son was mad, he went straight to his mother crying
Son : Mom, I am so mad at dad ! I fell in love with six gals but i cant date any of them. Because dad is their father
The mother hugs him affectionaly and said : My son , you can date whomever you want, He isnt' your father .