My mother remarried, and we got a sister who is a little older than me! I was happy to have a kind sister-in-law, but she was a lesbian who raped my younger sister at night! Our new house is small, so the three of us were sleeping together in a river shape, when we heard lewd voices from next door... I took a quick look and saw my sister-in-law attacking her! I was surprised, but I got an erection, and when I couldn't hold it in and started masturbating, my sister-in-law noticed...