My older sister, who is on her period, was in a bad mood as soon as I got home, probably because something unpleasant happened outside. She's usually kind, but today she's been picking fights with me and bullying me. She even teased me about my lack of interest in women, and suddenly started stroking my dick vigorously, spitting on it, and licking it. Despite feeling scared, my dick started to get hard in no time...