My girlfriend invited me to her house where I found her sister alone.

So I sat there waiting for my girlfriend while her unbelievable sexy sister was sitting next to me. A few moments later she whispered to me:
"We should have sex while my sister isn't home."
I immediately got up and turned around to head back to my car. I find my girlfriend standing by the door, she hugged me and said:
"You've just won my trust honey!"

Moral of the story: Always keep your condoms in your car.