She asked me to sleep at my apartment for one night, she was meeting with some friends and didn't want to travel by night. It was early in the morning and I thought my sister was still asleep but when I got to the kitchen she was already there only with a bra and panties, I was naked, as usual when I'm at home. Watching that sight made me want to fuck her, and she didn't oppose to it, so once again I fucked my sister.