It's been 3 months since my sister started living with her unemployed brother-in-law (husband) and my younger brother. I paid my sister a small allowance for doing housework. When I saw her comforting my depressed brother-in-law, I felt a stir in my crotch. While cleaning my brother's room, I lifted the quilt and my sister noticed something when she saw my erect cock. One day while drinking together...