My sister, who broke up with her boyfriend, went on a hot spring trip and asked me to go in her place. Turns out, we ended up in a mixed-gender open-air hot spring room! IENE-433

The older sister, whose boyfriend wouldn't go on the hot spring trip, asked her younger brother to go in her place. Since they were staying in a mixed-gender bathhouse, her brother's unexpectedly large breasts caused him to get an erection! When he tried to hide it, the older sister leaned closer… and then… looking at her erect penis…