One time I had sex with my sister.
"Oh, brother!" she said sexingly. "I am your sister and this is so wrong!"
I replied "Yes, I know, sis! But since we both came from the same mother and father, that's what makes this so sexy!"
She looked into my eyes and said "I lust for you like a boyfriend but really you are my brother."
I whispered throbbingly in her ear, "Yes. We are related."
Then we came and went to bed and mom and dad will never know.