Son: Hey day, I've fallen in love with an awesome girl.
Father: Thats great son, who is she?
Son: Its Sandra from next door
Father: Ohh...I wish you hadnt said that. I have to tell you something son, but you must promise to never tell your mother...Sandra is actually your sister.
A couple of months later...
Son: Dad, I've fallen in love again and she's even hotter
Father: Oh...who is it this time
Son: Its Angela from across the road
Father: Ahh...have to bear bad news son but Angela is also your sister.
Son is naturally well pissed and goes off to see mum...
Son: Mum, I am sooo mad at dad, I keep falling in love with great girls but I cant date any of them because dad is their father.
Mum hugs him affectionately and says...Son, you can date whoever you want, he isn't your father.