My half-sister had a tiny waist, firm ass. She had a similar black lingerie she wore when we were alone. I liked grabbing her ass, telling her she was hot. Then one night I told her I wanted to fuck her. She told me she would've let me if I wasn't her half-brother, so I brought a mask. The next time we were alone I put it on her, telling her to pretend she was someone else. We locked ourselves in my room and for the night she pretended to be the girl next door.