After her boyfriend canceled on her trip to the hot springs, her older sister brought her younger brother along because she didn't want to waste time. Since she was supposed to go with her boyfriend, they naturally shared a room and took a mixed bath. She gets an erection when she sees her big breasts, which she doesn't usually pay attention to! While she tries to hide it, her older sister approaches her, shaking her big breasts. She thought she would get angry if she was found out, but she couldn't help but stare at her erect penis...