Confusion

Son:"Daddy, I fell in love & want to date this awesome girl!"
Father: That's great son. Who is she?
Son: It's Sandra, d neighbour's daughter.
Father: Ohhh I wish u hadn't said that. I have to tell u something son,
but u must promise?not?to tell ur mother. Sandra is actually ur sister.

The boy is naturally bummed out, but a couple of months later ...

Son: Daddy, I fell in love again n she is even hotter!
Father: That's great son. Who is she?
Son: It's Angela, d other neighbour's daughter.
Father: Ohhhh I wish u hadn't said that. Angela is also ur sister.

This went on couple of times n d 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, u can date whoever u want. He isn't your father =))