My mother's remarried husband assaulted my younger sister, which made me see an erection. (Nanase Hina MIAA-229)

My mother's stepfather, who came from her remarriage, used the excuse of disciplining my sister to remove her underwear, leaving her completely naked. Although I was standing nearby, I was powerless to help. However, it wasn't that I lacked courage; rather, I wanted to see my stepfather violate my sister...