Two Of You

I was enjoying his cock when we heard the door slam and a female voice shout....
"I'm home!"
"Who's that?" He asked.
"That's my sister....don't worry she always stays in her room...."
"Your sister? I thought there were only two of you....so you have a brother and a sister?"
"No....just a sister....it's just the two of us...."
"So....that would make you her sister....I was sure you had a brother....."
"Look....I'm the one sucking your cock.....does it really matter if I'm her brother?"
"Oh no....not at all!!!!!"