I came to a hot spring inn with my brother and his wife. While we were on vacation, I was looking at my sister-in-law, who I found strangely sexy and excited. While soaking in the famous mixed-gender open-air bath, I was excited by my sister-in-law's body, which I was seeing for the first time. Unable to resist, I touched her body, and my sister-in-law, who seemed serious at first glance, was trembling with excitement! A secret sex session that I can never tell my brother about begins...